I like to re-open these issues, because they are not really solved:
Multiple signatures are indeed needed for some folks, e.g. one secretary writing for different legal companies. Such signatures may just vary in a few words.
R11 offers one native signature only. Additional can be selected by file, but that's not the intention for this feature request! Modifying (HTML) files is too difficult for many users.
It should be possible to select from a list of signatures available.
I like that suggestion best (from NTS-I-62, November 01, 2019 08:39):
Please let the signature follow the "Location document" in the personal NAB. Then it will be possible to have unlimit signatures.
But, of course, this could also be done in the mailbox file itself.