Both the hardcore and in-game difficulty modifier mods make enemies more powerful, but neither add more, to my knowledge. I think, though, that such a mod does exist. I can do q better search for one once I get back on my computer (I'm using my phone right now).
As for adding the enemies yourself, my understanding of modding - which admittedly is not as detailed as most other people in Holowon Labs - leads me to believe that you would need to extract and edit .utc files with KotOR Tool (the easy part), and also write new script files to both place them in-game and make them hostile (definitely the hard part). For more information on making such a mod or having it made for you, I suggest posting in the Mod Requests subforum of Holowon Labs.