Hi Mooshi, sorry no one responded to your post. I've been working with this lately. In SIE editor you just need to look for any of the .mgn/.msh files and extract those. Then there is a 3D@ script, and I think a blender? here on modthegalaxy that you can download and run in the 3d software. You can export your new mesh or amended mesh back into .msh/.mgn format with the same scripts. Just keep in mind these scripts don't reattach .mgn's back to their skeleton/animations therefore it's not entirely possible yet to make additions to .mgns. >msh a.k.a static type meshes can though.
I think I got that all right. I've been messing around lately with 3d modeling myself and have just made some client-side changes.