Each pack will often have some top level folder in it's archive, then inside that folder will be the portraits and bodies folders.  The portraits and bodies folders are what matter, that top folder does not. If you want to use multiple, you need to merge together the contents of the bodies and portraits folders.

I typically do this by extracting the packs in one location, then copying and pasting the bodies and portraits folders into "%APPDATA%\Strive", repeating for each pack so Windows merges them.