I Have an exterior door leading into my basement. It is in a cinderblock foundation. The door frame is built out of 2x can I rip it out and replace it with 1x pt to mount a new pre hung door? Thanks in advance.

Yes you can use a 3/4 board as a fastner, you may have to angle your nails to hit it. As long as the brick molding will catch the 3/4 board it will be fine