You need to link said improvement to either:

A) An Ability, Trait or Race Type

A custom Tech Tree (Yes still doable, the old system is still in place)

Mod  guides, tools and the likes are lacking, officaly and unofficaly. If you know how to mod and read xml and xsd files it will not be difficult to trail and error edits/additions. If not your in for a rocky road. Most modders (that are left) on these forums are quite happy to answer/help if you run in to issues, so feel free to drop in and ask.