Steam gift game on Origin

@fjfj00  To fully connect your Origin and Steam accounts, you'd need to buy an EA game in Steam, one that specifically supports account linking.  The games that do so should list that as a requirement on the purchase page.  For Sims 4 specifically, you'd also need to own the base game in Steam: there's no way to add your Origin-bought game to your Steam library.  So without purchasing the base game, you wouldn't be able to use the Steam-gifted pack.

For what it's worth, playing through Steam can be a headache.  You'd still be running Origin in the background, which means there are two game launchers that might malfunction instead of one.  And you'd actually have to uninstall Sims 4 and install it through Steam, then download your other packs through Origin.  So unless you really want to play through Steam for other reasons, it's probably not worth the trouble.