Adding new API methods in Nopcommerce Official API Plugin

I need to add some methods in nopcommerce official API plugin and I wants to customize some of the existing API's, but I am unable to do so, I didn't find any documentation which can help me.
How can I locally develop new methods and customize existing, I have the source code of the API plugin but I can't figure out how to use that source code in the existing Nop application.
Do I have to create plugin by using this source code of Nop API's?

Any help will be appreciated.
Thanks

You can refer that plugin other api and you can customize according to that
If still not work for you
Then please contact nop team they will sure provide support and solution for that