When choosing a pre existing case participant from the drop down list, there is the option of creating a new role for that participant, if they do not already have it.
For example, you could choose to populate the drop down with all case participants of type PRIMARY, MEMBER and ALIEN, and choose to create a new role for the participant of type ALIEN. Then, if the participant chosen was already of type ALIEN, no new role would be created, but if the type was either PRIMARY or MEMBER, it would.