Yes, this is possible! :)
Assuming you are not using the XSquawkBox plugin, all you have to do is enter the "Aircraft and Situations" tab from the "Aircraft" menu, and click on the "Other Aircraft" tab near the top. You should see a page with a menu that looks approximately like this:
- http://www.x-plane.com/manuals/desktop/images/combat/select_aircraft.png
You can use the up/down arrow to increase/decrease the number of AI aircraft. Select each file line to change the aircraft. This may take a few moments to load the aircraft selection menu, and then will take another few moments to load the aircraft once selected. Do not worry about aircraft team colors for now.
Bear in mind, increasing the number of AI aircraft will have a continual hit on performance the more aircraft you add. :)
Cheers.