Yes Uma, we should connect to Abap system and make it as one of the UME source for EP.
Once this is done the users created in abap system will also be available through portal and if u look at the groups tab of users u can find the abap roles assigned to user in the form of portal group.
This is portal group that i was explaining about. If you map a portal role to this group all the users with that portal group(abap role) will get assigned to portal role.
Regards
Sandeep