Say you find a cool hat skin and you want to use it in your game, but it's a mod for a hat you don't have. You can just change it so it's for a hat you DO have.
I really wanted the Courageous Conical, but it was for the sober stuntsman. I have ellis cap, so I changed it so it'd work for the ellis cap.
I'm going to explain what I did using ellis cap as an example. Heres what you do:
1.) download the skin you WANT.
2.) download a skin for the hat you HAVE.
3.) go into the folder for the skin you WANT, the folder is the one labeled model. In there is a folder named player. Theres more folders, but keep digging till you get to the files ending with .dx80, .dx90, .mdl, .phy, .sw, and .vvd.
4.) do the same for the folder for the skin for the hat you HAVE.
5.) Final step. Simply replace the name of the skin you want with the skin of the hat you HAVE (for me it was replacing stunthelmet.vvd with demoellis.vvd)
Basically, the game replaces not the stuntsman, but the ellis cap, because it was named as the ellis cap. It's all about switching names. To summarize, you just copy and paste the names.
Note: I only tested the ellis cap, other hats may or may not work.