Yes, you can. If you take the level files you have (not just the .txt files, but all map000xx files from your mod, simply replace the ones in the KeeperFX\levels folder and it will work.
And indeed, it is also possible to make it a separate campaign, so you can either play the 'all monks' campaign or the regular campaigns. To do so, make a folder inside campgns where you put your levels. And make a copy of the keeporig.cfg file in that same folder, rename it to something unique, and change it to fit your new campaign. In this file all options are described so this is easy.
One thing to note though, in KeeperFX a monk requires 8 tiles of temple to be attracted. If you don't want this, change the \creatrs\monk.cfg file. If you have a custom campaign you can give your campaign a unique creature folder as well.
Feel free to ask further questions, but this page will help as well.