This cannot be done.
However, the quickest way to view your skins is to fire up Il-2, go to the quick misison builder and assign the skin to an AI aircraft. If you hit fly, you can view the skin you made. End the flight, alt-tab to photoshop and make any changes you want. Save the skin and alt-tab back into the game and hit refly.
It is better to assign the skin to an AI aircraft. The game seems to remember the version of the skin you use on your own aircraft. If you want to view any changes in the skin you made, you must first take another aircraft for a spin or, exit and restart the game. The game reloads the .BMP everytime for AI aircraft. Photoshop does run a bit slower with Il-2 running in te background tough.
Happy skinning!