Only way I could think, to override that relation with some database tables, is writing like 20 different things in DataProvider... which basicly means also overriding stored procedures that are looking into roles etc.

So my solution was doing automatic synchronizing functionality for roles. Mapping users, roles and user roles between remote system, but only storing roles in DNN's own table. Which means, that there aren't any users in User table and no rows in UserRoles. I have no ambition for rewriting DataProvider, while in next version there can be changes that broke everything that I made on last week. Synchronizing is more or less stupid and annoying thing, but it was the only way I managed it in any reasonable way I could think.