Is it possible to add a category in the Civilopedia?

If it is, how do I add a new category in the Civilopedia?

Yes, with python. Check out Edgecrusher's Heroes modcomp. It adds a new category: Hero Units.

Technically, I guess it requires SDK work to interface correctly. I am still in the process of teaching myself this.

The stuff I did with python is more of a "work around", But it works.

Hope that helps.