Allow a client to issue access & refresh tokens (offline tokens)

Is it possible for a Keycloak client to be an application that issues access, refresh/offline tokens that are accepted by the Keycloak itself.

What I’d like to do is to add custom and complex meta data to those offline tokens & access tokens without touching any kind of Keycloak token mapper extensions, as they usually break once you have to upgrade your Keycloak to a newer version, which is a pain to fix.