If someone could tell me how to create a new XML table I'd be very grateful. I've tried before, but it didn't end up working ingame?

The game isn't "smart" enough to make your new table work automatically. To make a new table work ingame, you need to create the code that makes it work. Since we don't have access to the DLL code for now, the only option is to use LUA.

This mod (http://forums.civfanatics.com/showthread.php?t=395607) is an example of making a new XML table (MinorCivilization_UnitClassOverrides) that works ingame.

So, what if I want to hijack another table for something? Say I want a Policy effect in a building, or vice versa?

You can't do it, adding XML tags that don't already exist will have no effect, except if you want them to contain data used by your LUA script.

Aah, so there isn't a way to do it without getting involved in Lua. Thanks PawelS.