You can use alternate ring bearers as regular companions if you really want, but that really isn't their intended purpose. Just look at
Boromir, Bearer of Council. He has no signet, and only one real ability. The idea is that you make sacrifices (risky abilities, low resistance) for a better fighter, and the ability to start another big companion. For example, Isildur is a must-have in a Gondor Knight deck. As is Boromir, in a
Noble Leaders deck (start with him and Aragorn, or him, Faramir, and Denethor).
However, you can't have one ringbearer take over for another. In the case of
Sam, it was always "if Frodo dies, make Sam the ringbearer." A wise person once told me "always do exactly what the card says", and no version of Frodo or any other ring-bearer says to take over when another ring-bearer dies. In fact, if a ring-bearer is corrupted, (including Frodo) not even Sam can take over for him. That's game, no matter what.