The only thing I know is via Initialization Rule in BO Journal#Email like in the screenshot below. With a bit HTML code you can also include pictures. of course you can autopopulate Name, Phone Number etc.
I had that in use before we switched to add the signature on the Exchange Server when the email leaves the company.
Here you can see how I added email signatures (can be duplicated to add email templates too) to our environment.
Of course, in there you can add a rule to make a specific signature the default if that is what you want, in our case we wanted people to actively select the signature as in most cases the idea is to have the clients deal with "the Service Centre" instead of an individual.
If you have any question let me know.