Saving data

Is there a way in MCForge or FML to save extra player data on the server side, for instance whenever a player connects to a server it will autocreate a file for the player with all the data that it needs, like xp current xp level inventory items, ender chest items and all those things. But for a mod I want to save additional data to that file, but I have not found a way to do such things. I hope someone has an idea on what to do....