Ok, This is a Portal question.
There is no flag which is available for this. We can work around with populating other fields on the screen. In the Portal User Management Screen, after selecting a user, you should be able to see the tab "General Information". This has got a field called company. This field is mapped to User Group field under Logon data in SU01. Similarly in the Portal under "Account Information" tab, you will see Logon alias. This is mapped to Logon alias field in SU01.
At the time of Self-registration, you can get these fields populated by the user. Later, you can schedule a job which executes a custom program to check for this field and assign CRM roles.
But I dont understand if this is the right approach. Usually, if you are exposing the Portal, you create a BP and refer it to an Internet User and then request the external person to use this new login which we provide.
Cheers,
Murali