Adding a signature to Automator email
AppleScript
You can set the signature yourself with AppleScript:
tell application &quot;Mail&quot;
    
    set myMessage to make outgoing message
    set subject of myMessage to &quot;this is the subject&quot;
    set content of myMessage to &quot;this is the content.&quot;
    
    set message signature of myMessage to (signature named &quot;Default Sig&quot;)
    
end tell

See AppleScript/Automate Mail Tasks for sample AppleScripts to create, combine, and send messages.
Graphic User Interface (GUI) Scripting
While less reliable, you could try scripting the GUI to select the signature.