Velvet your doing it wrong. You have to go into the GCF (TF2 Content) , search the file name of the model, but put a wildcard for the extension (like rock001*). then, you extract those files, and also get the materials for them in the TF2 materials GCF.
You have to rename all of those to something,
but they must still retain the same character amount. like rock001 would become rock01a or rck001. Also don't forget the filename in the vmt.
Now you need a hex editor. I'm currently using
XVI. So you need to open up the .mdl file (like rock01a.mdl or whatever) and edit 2 things.
1. The top of it, you should see a filename. change it to the one you changed yours to.
2. near the bottom, you will see some material paths. particularly you need to change the thing after body, to the new filename you chose.
Then all you have to do is install the files in the corresponding folders. so lets say i took those rocks out of props mining (i think you did). you would go to the models folder in tf, then make a new folder called props_mining. put all of your files in there, save the vmt and the vtf. Now for the textures, you would go to tf/materials/models/props_mining same steps as before, and put them in there.
I was writing a tutorial on this, but my test model became invalid for reasons unknown, so expect a better write up in the future...