First you take your mask model in Gmax or 3DSmax, then you import one of the game mask models. Place your new mask in a good position, delete the games mask model, then link your model to the Aurora base. After that you can export your model and rename it something like: i_mask_017
For putting any custom model in the game, I think this tutorial should do it for you. There's a companion tutorial there that will show you how to make the item a new one instead of overriding a stock one. Hope it goes well!