Could you name an example file? Actually replacing .txd files would work indeed. I had swapped CJ's white tank top (which he wears at the game's start and on MTA Race) with my black Volcom t-shirt, cuz I disliked the stock CJ on MTA Race back then
. Extract the file, rename it to an available one and replace it. Perhaps the fitting .dff file needs to get replaced aswell (I think I did that back then to avoid bugs/crashes). However, I guess you're looking for a way to actually add those clothes to the Wardrobe or sth eh? The question is - were those clothes actually meant to fit CJ (?), if not you'd have to convert them (but I assume all player.img files are CJ related). Then they're just not listed, I guess thats upon some file which clothes are available on which shop & the wardrobe.