If you have this mod enabled, you will be able to use custom models you've downloaded in pretty much any game mode. To change it, you can go to sandbox and hold the c key then change your character or type in "playermodel_selector" (without quotations) into the console command to bring up the character selector in any game mode. It works in slashers but if you want the original character models back, you need to disable the mod first before you start up the game mode.

https://steamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=504945881