Adding a new web service function

Here at Tishreen University, We are trying to develop a new Web Identity for our university.
And for this purpose, We are using Moodle as the main system for our university LMS.
The problem I'm facing is that the main site is requesting some information from Moodle.
The existing Web Service API functions is not enough to satisfy the requirements.
I want to write a new function, for example I want to get all the sub-categories inside a certain category by sending the ID of the category.
All I need is to add a new function to the Web Service API functions inside Moodle.
Is there any way to do this? I mean to add a new function to the Web Service API functions.
The solution I found is to add a new plugin, but does the function which I wrote inside the plug-in is accessible from a web service?
Is this the right way to do it?
Thanks a lot.
Best Regards