When you work on your Livemark project there are different options of adding a plugin to the building process:
You can install an external plugin, for example:
$ pip install livemark-ckan
And activate it in livemark.yaml file:
ckan: true # or a plugin config
You can Write a Plugin and put it into plugin.py module or export from plugins package in your project root. You don't need to activate it in livemark.yaml. For example, you can take a look how it works in the Livemark Project Template.
It's also possible to add a plugin programmatically:
from livemark import system
system.register(CustomPlugin)