You can, yeah. Simplest way is to use the "Add Empire" button and add the empire's you'd like to face, then turn off the random generation. Make sure to edit them and set them to computer control.
Getting randomly generated AIs is a bit harder, and IIRC requires Gold.
code:
Minimum Computer Player Low Setting := 1
Maximum Computer Player Low Setting := 3
Minimum Computer Player Medium Setting := 3
Maximum Computer Player Medium Setting := 7
Minimum Computer Player High Setting := 6
Maximum Computer Player High Setting := 10
Minimum Neutral Player Low Setting := 1
Maximum Neutral Player Low Setting := 3
Minimum Neutral Player Medium Setting := 3
Maximum Neutral Player Medium Setting := 7
Minimum Neutral Player High Setting := 6
Maximum Neutral Player High Setting := 10
Those are the lines you want, they're near the bottom. Simply specify the same min and max, and you will always get a specific number of randomly generated AIs.