We framed in a bathroom in our metal barn. To allow plenty of room for installation and for added strength we used 2x6s.
Is it possible to adjust a standard prehing door to fit?
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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?
it's common to have to add jamb extensions for exterior doors on older homes, and I've had to do it a few times for interior doors as well.
Simply set your jamb with the hinge side flush to the finished wall, and rip trim boards to extend the jamb to the other face of the wall. If your jams are stained you would normally set those with an appropriate reveal, and not try to put them flush with the jamb faces. Flush joints usually don't look very good. If your jams are painted you could set them flush, but use a good wood glue to bond them so that the joints don't crack and show later.
Here's a representative example: