If they are rolled and have a stipple texture on them from the roller, it really won't matter how you do it. I still don't think you should practice on someone else's doors your first time out. I learned to spray by watching someone else as a helper for a long time and practicing on scrap wood/drywall etc.
The point is, if you've never sprayed before you could ruin a door with the first squeeze of the trigger. I'm not saying don't learn. I'm only saying don't learn on someone's else's stuff. Controlling the flow, overspray, etc takes time to learn. Practice so you can be confident your customers will get a professional result.
I'm with slick though....maybe since they are rolled allready, you can use a foam nap roller to do them again. You'll still be able to fly through them quickly, and you'll get a pretty smooth coat. Good luck whatever you decide.