You can download the code to the formation pack used in Brytenwalda toward the end of
this thread (the most current version at time of post is found
here) and add it to Native's source easily as it uses the
ModMerger Framework.
The self-contained mod Custom Commander adds some formation-like things, as well as a number of further modifications...but if you're just looking for Brytenwalda-style formations, the links above are just what you need.