This is mostly to work on my skills on making and editing models and textures, though I have instantly hit a wall. Sadly I mostly know Blender and don't have 3dxMax.
As I noted in a post by an admin two years back "Our artists use 3D Studio Max and Maya. Clyde (which, I should note, has moved to Github) includes exporters for Max." by Bifurcate a Lead Programmer.
I was looking to try and make several SK gear in usable reskins for Team Fortress 2, as well as take some enemy models and textures into new weapons. But I don't want to even mess with the texture files until I can see them on the models.
I even wanted to try and make a little SK source film animation. But again would need the models, touch them up a bit (As i noticed in some odd spiralspy java program that the hands and some things are rather flat and would need to be filled out more for an animation).
Is there some kind of plugin or tool so I could open up and use the .dat model and animation files for Blender?
I wish, I know 3ds max and would love to just drop in the sk models in it, I've only dug around the surface and by no means know anything about programming and the way the game is built, but it would seem you can't really, or no one has made it so. Maybe if someone dug around Clyde they could make it work, but I also wonder what OOO would think of it, I don't think they'd be happy about it. However you are looking for Blender, if there really is a way to export back to max then there has got to be a way to go from max to blender, I'm sure of it, they're both really popular programs. Also, your best bet should be to just re make the models yourself, or take your own spin on them, I remember seeing in SK's fb they posted a link to a fan made mod for skyrim or dark souls (forget what), which was a shield from sk, original model, slightly different shape and probably texture. They didn't mind that, hell they promoted it. Also, I support you, I like tf2's customizability, and would love more SK related content, so far this http://tf2.gamebanana.com/skins/128910 is all I've seen.