Hi @Skot ,
There isn't really an option to add a "custom field" in the user record in Canvas. It might be possible to re-purpose one of the existing fields, like "integration_id", but most of those fields aren't really visible to the user.
If I were trying to give students a way to check their account balance, I think I would add a link in the global navigation menu to let them go out to Banner (or wherever) and check their account balance directly from the source, especially if you're using the same SSO for both systems so they don't need to log in again. That way, they're checking their actual balance in "real time", rather than getting a value from whenever the user feed was generated.