Hi, I'm currently working on my second model for Team Fortress 2, some Texan cowboy jeans for the engineer, and I've run into a bit of a snag.

Here's a picture of it the model:
http://i.imgur.com/SqEiZrQ.jpg

Anyway, so the actual model I'm working on is just the pants part along with the belt, excluding the cable plug, tool pouch, etc., but now I've come to the stage where I'm finally done modelling and now I have to seperate the pants from the rest of the engineer model so that I can proceed to UV unwrapping. However, I've been thinking ahead and I've realized that I'm not so certain how I would replace this model over his default model in game. For instance, when people make hats and what-not for specific characters, how do they end up removing the characters default hat --something that's part of the entire character model itself -- and replace it with theirs? Being a noob at this and all, I would think that, for instance since the engineer's hardhat is part of the engineer model, you wouldn't be able to import your hat onto the engineer ingame without it clipping through his hardhat, right?

Alright, back to my specific problem, with these pants I'm making I just realized that this predicament is one that I'm going to have to finally figure out, since when I placed my jeans over the engineer's pants in blender there was a good deal of clipping. Anyway I'm not too sure at what stage I'm going to have to confront this problem, but I'd be much more reassured in my progress if I figured it out sooner rather than later.

So if the above is hard to follow, here's an idea of my question in a nutshell:
How do you replace a character's default cosmetic with one that you created?

Any assistance with this problem would be greatly appreciated. :)