I am currently using K2, I like the ease of the component.
Still I have noticed that it has certain limitations, for example:

It would be great to be able to add additional categories in the items in both the back end and the front end.

Note: I hope this suggestion can get to the developers of this excellent component.

Also appreciate if someone could tell me how can I make this request goes directly to the developer.

Hello,

There is an extenion for this called Additional categories for K2.

This extension works great when publishing from the back end but if published since the front end is not very functional.

I found the following extension extensions.joomla.org/extensions/extension-specific/k2-extensions/23653