How to transfer Mac Mail signatures between computers
A theoretical solution...

I find my Mac Mail signatures are stored in the folder at ~/Library/Mail/V6/MailData/Signatures. You may be able to copy the contents of the folder at this location from one computer to another.


Hit ⇧⌘G (or Go > Go to Folder... in the Finder) and enter ~/Library/Mail/V6/MailData on the computer that has the signatures you wish to copy. Locate the "Signatures" folder, and copy it over to your target computer.
Then (on the target computer), navigate to the same ~/Library/Mail/V6/MailData location, replace the Signatures folder (if present) with the version you copied. 
Open up Mail and check if the signatures are there.


Two warnings:


I haven't tested this, so it might not work/it might mess things up. Make sure you have a backup copy of the signatures folder from your target computer before overwriting it so you can revert if anything goes wrong.
If there are already custom signatures on the target computer, they will be overwritten.