Is it possible to install a retail copy of a dlc, in my case the arrival, to a steam version of mass effect 2?

What do you mean a retail copy of the DLC?



All DLC is downloaded from BioWare's website and yes you can install them on the Steam version.



The DLC is linked to your EA account (the same account as your BioWare Social Network account), so as long as you log into that account in game it will authorize it.