Adding MCMMO is a little bit difficult. It is plugin for bukkit server and only for multiplayer. There is no version for singleplayer. If Hexxit is based on bukkit, then you could be able to add mcmmo by just adding mcmmo.jar into plugins directory of the server. If Hexxit is not based on bukkit, then you can only take bukkit server and try to add mods similar to Hexxit modpack. But you will have to troubleshoot everything from scratch. Still after this action, people will have to have same mods as on server.