Be sure to include the new NFA item(s) to the trust before mailing in the forms and the copy of the trust.
You can always add items to the trust just by adding them to the list; cars and real estate property require additional steps.
You can also remove items from the trust; other than NFA items. Never remove them until they are sold and the new owner has his/her stamp in hand.
You can also grant others access to the items by making them trustees of your trust; even if only for a temporary basis.