I was damn certain that I had already answered this, but... try the Retrofit mod. Grand Campaign with Kingdoms' engine's features.

If you want JUST the boiling oil and no other Kingdoms extras you will need—at a minimum—descr_walls.txt and descr_boiling_oil.txt. You may need additional files for localisation and the like. Note that there seems to some changes between M2TW and Kingdoms which could necessitate modifying more files to fix compatibility issues. It's probably not worth the effort—try the Retrofit mod first and see if you can save yourself the time.

Incidentally, modifying the towers will also require changing descr_walls.txt.