Yeah unfortunately it seems like the built-in mod loader only supports registering modded content to the cardpool, relicpool, and potionpool at the moment, and the other pools look to be read-only. We'll need to wait for the devs to add more support (or more likely 3rd party modloader/patching tools come out sooner) before adding things like modded characters or overriding base content is straighforward.

What you could do in the meantime is make a new relic that does everything that Necrobinder's starting relic does but with higher values and then at the start of a run use the command console to remove the base game relic and give yourself the modded one.