Hello Reinhold,
Adding godparents relationships with people in Gedcom (or Ancestris) consists in adding a RELA tag between them. You do this in Ancesrtris using the Cygnus editor, in the bottom of the screen of the godchild. You click the "chain" icon, choose the event, associate the godparent individual. That's it.
Using specific home made tags is possible, using the gedcom editor, but not recommended. It will not be understood by most genealogy software.
Best regards,
Frederic