You may need a tool like
Hex Editor. This is used to edit the
.mdl files.
The server skins model generally have a custom folder name or custom file name for the skin. You need to change that custom name to client side supported name in the .mdl files using Hex Editor.
After that you need to also modify the path or folder/file names in
.vmt file. Just open the .vmt file in Notepad++.
In some cases, you may also need to modify the textures using VTF Edit/Photoshop/Gimp etc. But that is mostly not required. The above steps should work.