How can I use a standard prehung interior door on 2x6 studs?
You can buy pre-hung doors for 2x6 studding. If you already have one for 2x4 studding then it could be adapted, but it would be better to return it and get one designed for 2x6 studding. See here. Some doors with a split jamb might fit either one.

Do you already have the door on site?