I believe that, if you have bought your tickets from Virgin Holidays, you will not be able to add them to your MDE account prior to your stay, since Virgin don't issue real tickets, only vouchers, and vouchers cannot be linked to MDE (with the exception of AP vouchers). As far as I understand it, this means that you won't be able to pre-book FP+ for your visit but should be able to book them once you arrive and have checked in.
Unless anybody knows any different