I don't think there is a way to have a zero build time. I can think of a LOT of reasons why this would be....

1) The AI..... what would stop it actually making a mass of peasants in no time at all.....

2) The AI..oh...I mentioned that one

Anyway.... there is probably a much better way of doing this.

Go to the campaign folder and peruse the mercenaries text file. You could quite easily make a 'mercenary' peasant unit with a nice cheap cost. Next, make it available wherever you want to have it. I suspect the only reason the game doesn't have this sort of unit in place is that no-one would ever want to pay the mercenary cost and upkeep for cannon fodder :)