Ah, I think I've found a solution, over at gamefaqs.

"Quite simple, and effective way to give yourself an extra 1000 or so
years/turns so you don't have to rush or worry yourself. After all, Rome
wasn't conquered in a day! Neither will be Europe! Hellz no!!!

Open up the period textfile you want to play, at the top there is a place for
Start Date, simply delete the last digit of each year. For example, in
High.txt, it's like this:

SetStartDate:: 1205
SetPeriod:: HIGH
SetEarlyPeriodStartDate:: 1087
SetHighPeriodStartDate:: 1205
SetLatePeriodStartDate:: 1321

Right? So change it to this:

---> SetStartDate:: 120
---> SetPeriod:: HIGH
---> SetEarlyPeriodStartDate:: 108
---> SetHighPeriodStartDate:: 120
---> SetLatePeriodStartDate:: 132"

I am yet to find out whether this will work in the middle of a campaign or when beginning a campaign. I'll try tomorrow though, as I need to go now.

By the way, there are some good things over at gamefaqs. Somebody made a guide of EVERY SINGLE UNIT in the game, and wrote EVERY SINGLE THING there is to be known about each unit. Believe me when I say it is a long page. It'll take 5 minutes to scroll down it! I'm concerned about the guys health actually...