I bought the Sims 3 and multiple expansion packs at a store. I wanted to install some expansion packs through Steam, and I am completely aware that if I want to to that, I will need to first install the base game through Steam first. But will I need to install all my expansions again too? Despite the Steam Summer Sale currently happening, it would still be a ton of money buying them all again.
You can try redeeming your keys (official instructions here) first. If you can redeem them there, you will not need to repurchase everything on Steam. However, if you cannot redeem the keys on Steam, you almost certainly would need to repurchase all of the DLC on steam as well.