Installing retail expansion in steam?

[Scotsmania]

As per title, is this possible? I have the game Silverfall on Steam but bought the expansion for it a few weeks ago on Direct2drive. It's a 7GB download so i just want to check that i'll be able to add it to the main game that's installed through steam before i download it, if it won't i'll just get it again but on steam this time.

Tried adding the retail key it gives on Direct2drive to steam but it comes up as invalid.

 

[Lagmeister]

You can't in this case the only non-steam games that you can physically add to your steam account are on this list
https://support.steampowered.com/kb_article.php?ref=7480-wusf-3601

 

[Scotsmania]

Damn, that's annoying. Cheers for the info and the link though, i never usually buy anything on direct2drive but it was on sale when i did. That'll teach me to stick with steam, now i have to pay double instead of saving a few quid lol.

Edit - Anyone want a free retail key for Silverfall -Earth Awakening expansion then? The Direct2drive one is useless to me now It's a standalone expansion so the original isn't needed but you'd be better off if you have it so you don't jump in at level 45.