If you are after plot reasons then you could have them being an escaped slave (or descendent thereof) who wants to give back to the Federation for rescuing them from the Romulans.
Another possibility is that due to political games high up there was an agreement to do an officer exchange between Star Fleet and the Romulan Navy. Some in the Romulan Navy objected so sent a Reman as an insult. Politics changes and the Reman is no longer wanted back so transfers formally to Star Fleet.