I heared it should be possible to add a custom skin to a model by just modifying the fgd file. Is this true?
No, that’s not true at all. You either need to decompile the model, adjust the QC file to add an additional skin, and then recompile it with the studiomdl tool, or you can overwrite the existing skin with your own.
Can’t you just like, hex edit it?
Copy paste all the files related to the model and textures.
open a hex editor and rename it and change the associated paths.
Save.
Profit.
It’s actually a bit more complicated, but I can’t be fucked telling you because you can’t be fucked using google.