Pretty easy.  In the flight plan .txt file at the end of the outbound and endbound strings and just before the airport designator, there's an option to set the flight number to anything you want.  In your case, you would need ten different flight plans (or five, if you want to-and-from the same airports), and set the flight number to whatever you want.  BE SURE TO SET THE OPTION IN THE FIELD PRIOR TO THE FLIGHT NUMBER TO "F'.  If it's set to "R", ATC will call the flight by the tail number.

Suggest you download a great tutorial by Craig White called "Adding AI Flights in in FS2002."  It is well done, thorough and easy to follow.