As far as I can tell, this is not a simple thing. Haptek apparently have some tools for converting from various standard 3d file formats (such as Maya, 3D Studio Max, Milkshape) into Haptek formatted files. This is part of the collection of tools at haptek.com, but you would need to purchase them from Haptek to make it work, as they offer a contact link, but no demo downloads that I can see.
If you can get the Halo character model in one of these standard formats, then it might be possible, but will probably require quite a bit of work, and may require purchasing tools from Haptek. I suspect that the Halo characters are probably not in a standard format anyway, most game companies create their own formats for storing data specific to their games.