It works only on unpure servers, because it obviously can be used to gain an in-game advantage. Just extract the models (.md3) files from the data-1.1.0.pk3, edit them as you wish and put them in a new pk3 using the same file names as the original models. If I remember correctly the game will pick the models from the last pk3 first, so you should name it zzz-mymodels.pk3 or similar (maybe I got it wrong, then use aaa-mymodels.pk3).