Is there a way to add a new/modified translation to an existing TM?
I have a modified target document though do not have the original source document on hand.
Do I understand correctly that your TM contains the translation units of a predecessor document from language A to B, and now you have a modified document in language B and want to "reconstruct" the "source document" (language A)? In this case you could use AnyTM to "turn the TM around" and load your modified document in a project with source language B and target language A.
Did I understand your intention correctly?