When you want to use self-assign for some shoppers, but still allow others to apply, you can set some preferences in the Profile.
By setting a minimum shopper rating of 9 and checking the box for "Preferred/Qual Self Assign", the profile will allow shoppers with a 9 or 10 rating to self assign, and only let other shoppers apply, not self assign.
Once you have set up this profile, use it to create a session as you would any other profile.