Yes it's possible. Little technical though but if you check out some of the tools for AI flights you can probably work it out. Just beware that some planes take a bigger toll on your system's performance. If you use those for AIs then you'll also have to pay the price in FPS.

First off you'll definately need TTools. It's a program to decompile your AI flight plans into text files. I've seen several programs to create AI Flight plans. I haven't messed around with them much myself but choose whichever seems easiest for you to use. I've made one or two AI flights with a program called "Simple AI". I've ran into a couple bugs with that one though but it works OK sometimes. Just make sure you save a copy of your AI Flight plans before messing with them.

If you are really into coding stuff you can also add AI flights to the text file created by TTools by hand but that takes some knowledge of how they work. Little too technical for myself for just a couple flights I wanted to add.

Just search here at SimV and over at AVSIM and you should find several tools for adding AI Flights.