Under Technicians from within an instance, you can only modify users as technicians whom you have already imported under Users.
Select users from the Change as Technician drop and click on users to modify them as technicians. Once you click on the user’s name, technicians details page will pop-up with the selected users' data. You can only update the Associated Groups, Cost per hour, Project Roles fields, and additional fields added from the instance. Click Save to change the user type to a technician.