While it is difficult to offer a response that is 100% accurate in the absence of further detail about the particulars of the case and especially the synagogue, the likely answer in most cases is that it will not be allowed. As a result of the differences between the denominations in terms of what constitutes the acceptance of mitzvot in conversion, as well as issues surrounding who is qualified to serve as a member of a Beit Din to witness and attest to a conversion, the orthodox community does not accept non-orthodox conversions as legally valid. Thus, an orthodox rabbi could not marry a couple in which one or more members of the couple had a Masorti conversion, due to issues of Jewish status. However, there is a phrase in the question that is vague and may present a possible alternative conclusion. Until this point, the words “celebrate a Jewish marriage ceremony” have been assumed to mean having the wedding at an orthodox synagogue, and presumably officiated by an orthodox rabbi. This is unlikely to be workable. However, if the phrase refers to having the celebration dinner or party at the orthodox synagogue in their social hall, the answer may vary with the policies of the synagogue. Some synagogues will not allow a celebration to be held on site that does not conform to orthodox halachic (legal) standards, due to concerns of what is termed “mesayin ovrei aveirah” – adding and abetting someone to do something that one believes to be wrong (see Talmud tractate nazir.) However, others rent out their social hall for all different kinds of community events without necessarily approving of the nature of the event. As a result, an orthodox synagogue social hall may, depending on synagogue policies, be available to you so long as standards of kashrut are maintained so as not to damage the kosher status of the synagogue kitchen.
The short answer to this question is: it depends if the rabbi of the Orthodox synagogue considers the conversion valid.
There are several requirements for a conversion to Judaism. They are mikvah (ritual immersion), brit milah or hatafat dam brit (circumcision or taking a ritual drop of blood from a circumcised male), and kabbalat ol malchut shamayim (acceptance of the yoke of the kingdom of heaven). Men are obligated to fulfill all three requirements and women, clearly, are exempt from brit milah or hatafat dam brit.
One of the biggest differences between an Orthodox standard of conversion and a Conservative one lies in the definition of kabbalat ol malchut shamayim. This concept means that the individual undergoing the conversion must spend a period of time studying the Jewish tradition and be prepared to take on the obligations and responsibilities of a committed Jewish lifestyle. But what exactly does that mean? Does the person have to pray three times a day, observe all of the Jewish holidays, observe all of the laws of Shabbat, and commit to keeping kosher to one specific standard? In short, what is the "check list" for determining whether or not one has "committed" themselves to an active Jewish lifestyle? What defines kabbalat ol malchut shamayim? This is the main question that may lead to a difference of opinion between a Conservative rabbi and an Orthodox rabbi on whether or not the conversion is valid.
Another difference of opinion about the conversion's validity may center around witnesses. The qualifications for who can be a witness to the mikvah, and who can sit on the rabbinical court might be different for an Orthodox or Conservative rabbi. The qualification of a witness (their level of observance, their gender) might call into question the validity of the conversion for the Orthodox rabbi. Additionally, as we are talking about an adult, there may be differences of opinion surrounding who is permitted to perform the circumcision (if needed). Does it have to be a mohel (one who does ritual circumcision), or can it be a doctor who is Jewish and well versed in the laws of ritual circumcision?
I would imagine that each of these questions (and perhaps a few others) would be important to the Othodox rabbi because he would have to determine whether or not the convert asking to be married was Jewish according to his understanding of Jewish law.
Needless to say, this is an extremely sensitive issue for all involved. A Conservative rabbi may have questions about a Reform conversion much as an Orthodox rabbi would have questions about a Conservative conversion, I understand the challenges of this issue. I would hope, however, that this decision would be handled with the utmost sensitivity, care, and with an eye towards striving to bring this couple closer to Judaism and a Jewish lifestyle -- something they clearly value. In a time where there are fewer couples who even think to ask to be married in a synagogue, all of us, regardless of denomination, should approach these tasks with integrity and sensitivity as we seek to bring Jews closer to their Judaism.
In truth, the answer will depend. It will depend on the standards and requirements for conversion of the rabbi and members of the Beit Din that conducted the conversion, what requirements and rituals were included in the conversion, what the intention of the parties was and is, what the observance level of the couple was and is, if the Orthodox rabbi (and Orthodox community) accept the witness and members of the Beit din, and many other factors.
There is no hard and fast answer, but most often, I would guess, the Orthodox rabbi and community would say no out of a sense of caution or concern about the standards used and whether those standards met the requirements of Jewish Law (Halachah) as that Orthodox rabbi and community understand and follow them.
There may be exceptions, or special circumstances, but if you wish to avoid being refused, you would probably be best served by approaching a Conservative, Reform, Reconstructionist, or other affiliated congregation.
Of course, I hope that you understand that I am answering as a Reform rabbi, and cannot speak for rabbis from the Orthodox stream of Judaism, but I would guess the answer in the situation you describe is likely no.
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