I'm not 100% sure this will work, but as long as you can single out the rpa file for the Step 4 DLC, you should be able to move it to a different version of the game. I don't know where to find this file on Mac, but it should be called "dlc_wedding.rpa". If you can find that, you should be able to move it to a different version of the base game and still get access to the content. See if you can locate it, and hopefully I can help you further!
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I found the local files (from the other games I own) now but since the dlc on steam is deemed as incompatible for Mac, it's in my library and it's not on my local files, it's like not downloaded (and there is no option to do it), it just stays on my library. Thank you anyways, it was my fault for not checking the compatibility on steam, I'll buy it again here some other time