I'm not sure if you can, but are you willing to take the risk? All it takes is for a minor emergency to redirect the flight to another nearby island and the plan is gone. You're fiancee would be stuck outside the country and would have to fly back home, and wait their immigration journey out there. Imo it's not worth the risk. You could always have a small court house wedding now, and have a big, formal event in the Virgin Islands when your fiancee has a greencard.