The easiest way is to bring the door back & get one for a 6 inch wall
Other way depends upon the issue
I have a door for a 4" wall in a 6" wall on my front 3 season porch
It just doesn't open more then maybe 135 degrees
But since there is a storage bench/seat it doesn't need to
First make sure that the door frame is all the way toward the inside of the wall so that the door opens all the way. Then rip some wood to the size you need to extend the jamb. Have your siding guy cap the door jamb so that you cannot see where you extended the jamb. We do this all the time on jobs. The only down isde to this is that the threshold will not extend all the way out. Hopefully that will not matter in your case.