Geany offers a plugin API to access Geany functions from a plugin.
The plugin interface can be used to implement features that do not fit into Geany's core.
How to start writing a plugin can be found at http://www.geany.org/manual/reference/howto.html
The most recent documentation of Geany's plugin API can be found at http://www.geany.org/manual/reference/
A table with the API and ABI versions for Geany-releases as well as versions of various dependencies can be found here.