Buy The Sims 3 DLC and activate on Origin?

@zackerthescar There were problems between Ea and Steam from the very beginning. It really wouldn't be a problem if only the base game was the issue but Sims franchise has expansion packs (dlc) released over time. When TS3 was released the DRM was SecuROM and SafeDisc. Then EA created Origin which is a Steam like entity but even then EA did not change the DRM in TS3. Then comes Windows 10. Windows 10 does not support SecuROM or SafeDisc. EA had to change the DRM and this created even more confusion. If you want to be up to date and add new content (dlc) your game now has to be activated through Origin - it will continue to play if you don't try to add any DLC. If I remember correctly there was a breakdown between EA and Valve (Steam) in negotiations about TS3 DLC. and at some point Steam did not offer DLC after a certain pack (not sure on this). IMO I would not use Steam for a game that continues over a long period with DLC packs. The Sims 3 is property of EA and if a third party vendor does not agree with the terms of EA then it is the right of EA to deny the third party vendor. I know this is a long post to explain a simple problem but as you see it is a complex situation. I use both Steam and Origin - Steam for Tombraider and Fallout 4 - Origin for The Sims games and it works just fine for me.

add: to answer your question - if you get a code from Steam you should be able to redeem that code in Origin. Origin keys do not work on Steam.