Losing existing roles

I have a user which is assigned a special role, such as 'admin' - this is not a role that can be controlled through roleassign.module.

If a site moderator (who has roleassign permission) then edits my 'admin' user and grants them another role the, the user then looses their other roles which roleassign doesn;'t have control over - effectively removing their 'admin' role and loosing the control of the site!