This is a semi-educated guess
.... I assume you are the Grantor. Correct? If so, you should be able to just add the supressor to the list and sign and date beside it. As far as I know it does not have to be renotarized and no winesses or trustees are required for a Grantor to add property to the trust.
So, if I am understanding you correctly, you filed the Form 4 without the item to be transfered being listed on the Schedule A as property of the trust, and it was approved? There has been much debate here about when NFA items should or need to be added to the Schedule A. Logic and your situation, would tell me that my AR15 lower, or whatever, is not an NFA item or property of the trust until the form 1 has been approved and it is made an NFA item. Yes, much debate on this subject.....