I found an easy cheat (if you want to call it that) is to up the numbers beside the building types in the campmap-starpos -late era (for example) text file. If it says Castle 10, change the 10 to an 11. That sort of thing. I usually change the Town Watch to 4 because I want pikemen in Wessex right from the start. Just make sure the numbers you input actually exist in the game.

Another thing I do, is change the amount of time it takes to build something. If it takes 2, 4, 6, 8 years to run through a progression of building types, I change it to 1, 2, 3, 5. Something like that. But I'll up the cost to help balance it out and not make it too easy. So I get a unit twice as fast, but it costs twice as much.

I did add buildings by simply writing in another line of text in the building list and adding the building I wanted. This might not be the way the expert mods do it, but so far it works for me. No program required and it takes about ten seconds.