Create dovecot master user with submit authorization

Question

I'm having trouble in creating a master user in the dovecot server that is also a submit user (so it can use the urlfetch command).

This user needs to login as a master user so it can assume the identity of any other user but in this process it needs to keep its authorization to use the URLFETCH command (I don't care if this is the only thing that it can do).

I can create a normal user and configure it as submit user and I'm also able to create a master user, but not one that is able to login and change its identity but that keeps its submit authorization.

Is this possible?