Sounds like you might want to limit the portion of the helmet that you want to work with in Creo.
Hopefully you have some sort of software that will let you load in and manipulate the scanned data. If so, you might want to think of some of these ideas to cut down on the size:
(1) Is the helmet symmetric from left to right? If so, and if the items you want to add on are going to be symmetric, you could split the scan in half and only deal with one or the other side. You could mirror the geometry you create in Creo to get the other side, once you're happy with things.
(2) Do you know what specific region you want to attach to? If so, maybe do some further editing to limit the scope of the reference data to only the areas that will be needed. This can cut down on a huge amount of data.
(3) If you can generate such data, maybe look into creating parallel section curves. You could use the points from such curves to build splines, then build surfaces in Creo. I've done this kind of thing for junky surfaces that overlap or have weird discontinuities. It works really well as long as you're not looking for super-accurate geometry. It's not like you're making a prosthetic leg or something...
Dealing with 3D scanned data like this is something a lot of different scientific disciplines are doing these days. If you're at a university, maybe someone could help you in archeology, biology, or any other department whose research involves looking at the structure of things. If they're scanning bones or artifacts and comparing them they must be able to manipulate the data somehow.
