Steam – activate a DLC I bought through Steam on a game I bought through App Store

It will put the starter pack on the first account you login to from the steam version of the game, so you have to download it from steam and login there to get it, but after that you can go back to using the hirez launcher in you want.

No, it isn't possible.

To activate a game on Steam that you purchased from a third party, two things need to happen:

1. You must be provided with a CD key from the third-party retailer.
2. It must be a game for which Steam accepts third-party CD keys.

When you buy a game (or any other software) on the Mac App Store, you aren't provided with a CD key, since the MAS itself handles product activation. (Apple's requirement is that all software sold on the MAS uses Apple's activation methods. CD keys and the like are disallowed.)