I was having a look through the FSW installation folder, and I must admit that it does look possible to add other 3rd party addon aircraft. Because the game is different, I'm sure there could be some interesting/strange results.

I have just finished getting Windows 10 reinstalled, and working properly, on my computer, so I'll take a look at adding a simple 3rd party addon, before I try a Captain Sim aircraft (if at all).