Hm. Without hacking up some PHP code, this isn't possible from Register Plus Redux. What you could do is create an additional field for "requested role", in the options you would have to type up the roles you specified in the Members Plugin (manually, there is no way to link this, so if you add new roles, you would have to edit this again). In addition, you enable Admin verification of new users. Now, when a new user is created you can review the request, approve them, then change their role to the role they specified. That's quite a bit of overhead for the site Admin, but it's the best I can come up with without writing custom PHP.