[keycloak-user] overriding built-in verify-email RequiredAction

[The Fredo]

Hello all,
I need to extend a bit the built-in VerifyEmail RequiredAction (i.e. adding a specific attribute in the LoginFormsProvider, so that the login-verify-email.ftl template displays accordingly).What would be the right way to do this ?
I tried to build a new RequiredAction based on the existing VerifyEmail RequiredAction, but it happened to be far too complex for me.
(The only way I made it work was modifying the existing code (of VerifyEmail.java), it allowed to test that my data are available for the freemarker template).
Thanks in advance !
regards
Keycloak 3.2.1