If I currently own NFA firearms, can I transfer them into the trust without incurring an additional transfer fee?

Unfortunately, the answer is no. The ATF considers the trust as a separate legal entity and the tax must be paid for the additional transfer. Given that this is essentially an tax by the IRS, it would seem logical that the tax would be waived because a revocable trust is a pass through entity and not taxed. As far as I know this has not been challenged because it is only $200 for the transfer and the cost to fight it would be many thousands of dollars.