"mod" pretenders/nations set to AI after start ?

[Sombre]

In short - yes.

As long as you know which magic sites and pretenders they have you can create a mod which alters their properties and gives the AI a bonus. For example you could add

#res 100
#gold 500

to an AI controlled site and as long as you had the mod file loaded from the beginning the changes would take place in the save game.

By the sounds of it you might have already started the game, in which case I'm sorry but it's too late. You need to have a 'blank' mod loaded up when you started the game, ready to have extra bits input when needed.