Adding boiling oil ?

[Akka]

Ok, as surprising as it is, it seems that nobody asked this question (anyway, did'nt found anything with the search, so...).

I would like to add boiling oil in the MTW2 Grand Campaign. I know that there are mods which add it, but they usually come with other modifications, and I strictly want JUST the boiling oil, if possible by modding it myself.

Sooo... how can I do ? :x
I do have Kingdoms, but I don't know how to make the boiling oil usable in MTW2 campaign. Could someone enlighten me ?

Additionnally, I'd like to know where I can alter the power/fire frequency of the wall towers. I'd like to make them a little bit stronger.