Aloha Melissa,
It can be done but it isn't without it's challenges. If it's their old roof to wall lacing system then you'll need to cut the lacing on either side of the intended door and tie it off. The roof is not designed with a door cut out or rain diverter if you're adding the door. You'll also need to cut the walls, wrap the rope into the ends of the walls, chemical weld that seam and get the trailer molding if you want to somewhat replicate the way they connect the walls to the doors. Maybe they could send you a kit with H-66, the rope, and trailer molding? Give them a call, I'm sure they'll be receptive to giving advice and assistance. Good luck!
PS - A window is MUCH easier to add.