Re: Transferring from Origin to steam for the sims 4 plus expansion packs
@AngusZarco This isn't directly possible, as in, you can't add your Origin-owned content to your Steam library. What you could do is buy the base game in Steam, install it from Steam and then download your Origin-bought packs as well. You'd also be running Steam and Origin at the same time while you played, which is true for any Steam install.
By the way, if you did buy the base game in Steam and then decided the setup wasn't working for you, it would be difficult to revert to an Origin-only install. So it's better to be sure this is what you want before trying it.