Adding a signature to an XFA-file

Hi, I'm trying to get an official document signed by the boss, who is elsewhere. It's an XFA document, which means I cannot use the standard "ask for signature"-functionality. However, the signature bar seems to be in text format only. So I can type the boss' name, which seems a bit useless.

The only way I can seem to get this doc signed is by printing it, signing it and having it scanned again. However, the one I need to sign it is not present. What do I do? Is flattening it the only way?