Do you mean you bought the DLCs on Steam? If so, I should be able to help you! The DLCs are rpa files that you should be able to move from your Steam version of the game to the itch.io version, even on Mac.
In Steam, right click on Our Life, go to the "Manage" section of the drop down, and click "Browse Local Files". In that folder, click on the "game" folder, and you should see the DLCs in there labeled "dlc_step_1.rpa", as an example. Move those from the Steam folder into the "game" folder of the itch.io version.
If that isn't what you mean, just let me know! Hopefully I can help, and thank you for playing.