Well I found out how to do it yesterday, believe or not if you know how to make new armor, it works the same way here is a tutorial talk about that a bit:
http://www.youtube.com/watch?v=Anfqvte_px4
However with crest all you need to work on is just the diffuse texture. Make a new texture when it comes to the diffuse, then with a Hex editor open the rigid_model_v2 file. Then type in the new diffuse texture in the diffuse section of the model, here are two tutorials talking about that:
http://www.youtube.com/watch?v=_HJ-v72mXiM (start at 5:49)
http://www.twcenter.net/forums/showt...iginal-texture
Then once done with these you make a new variant definition for your new crest. You can add this to your custom or already existing helmets. Is there a limit to how many crest you can put on a model? That I don't know. I see the it shows alot of my crest, but I have noticed some are missing. So That is what I know about that I hope it helps.

God Bless.