@xaviorskailuuuu You'll need to buy your DLC again if you want to use it in macOS, unfortunately. The Steam version of Sims 4 is Windows-only, and that applies to the DLC you own there as well. There's no way to transfer it to the EA App.
If you do want to rebuy some packs, you'll need to unlink your Steam and EA accounts in order to have the option. The EA App will otherwise block you from making the purchases. An alternative is to create a new EA account and buy the content there.
I don't know what Mac you have, but if it's an older Intel model, another option is to Bootcamp it, i.e. partition the drive and install Windows on one side. This isn't possible on ARM Macs, at least not now. And of course you could keep your old Windows PC just for Sims 4.
