Hi and welcome!
Just to clarify, do you use Paintshop or Photoshop?
Now to the point of your question. I'd say making a paintkit from a repaint would be possible depending on the repaint itself. Very simple solid schemes or bare metal would be easiest to attempt this with though. The big problem is getting the aircraft detail elements like rivets and panel lines separated from the paint, and a simple monotone scheme would really help.
If you are able to get the important parts of the aircraft off the existing repaint, you'd then have to make them all a neutral color tone, like gray or black, as they'd probably be tinted to blend into the repaint. And from there you'd try to blend these details into your own work.
Now a final and most important consideration, is whether you'd intend to release this paintkit, or any repaints based off it. By nature, your kit will contain the work of who ever made the repaint you started with, and you would really want to have that artist's permission to redistribute a modified version of their work.
Hope that gives some answers. If you've any other questions feel free to ask!