Well technically for personal use you can load whatever tilesets you have/want into it, but in the end the tileset needs to be in a format/layout that RP can use, and correctly added to the config file. Cogmind tiles are not in such a format (I don't actually use them in RP myself, either--never have! I work in ASCII only).
The manual describes how to add custom fonts, and there are many on the website as well. Custom tilesets are essentially no different from fonts in that regard.