Re: Have Sims on Origin: Want to use my Steam Client: How do I link my account?
@joeygates27 The only way to link Origin and Steam accounts is to buy an EA game in Steam, and specifically one that requires that the accounts be linked. In your case, you'd need to buy a copy of Sims 4 in Steam, uninstall the game in Origin, then start to download the base game in Steam, which should prompt you to link the accounts. Once the Steam version is installed, you could download the packs from Origin, and they should work with the Steam install.
I think this should also work if you download Sims 4 from EA Play, but I've never tried it or heard from someone who has, so I don't know for sure. The problem is, once you add the Steam version of Sims 4 to your Origin account, it overrides the Origin version. Then if you remove the Steam version, Origin will act as if you don't own the game and will block you from playing. This happens all the time to people who buy and refund Steam copies of Sims 4, or who take advantage of a free trial from Steam. To get around it, you'd need to renew your EA Play subscription or buy a standalone copy of Sims 4 through Steam.
Just so you know, Origin will still run in the background when playing through Steam. There's no getting around that.