@racheljersey The answer is... probably, if you have an Origin copy of the game as well. I don't know whether this would work if you didn't own the Origin version as well.
When you buy the Sims 4 base game through Steam, you can also use any packs you've bought through Origin. When you register a disc copy of a pack in Origin, it gets added to your game library the same way it would if you'd bought directly through Origin. So since disc and digital purchases are treated the same way by Origin, it seems likely that Steam would treat them the same as well.
The catch is, I don't know whether you could register a disc copy of a pack in Origin if you didn't specifically own the Origin version of the base game. I believe Steam itself won't accept disc codes, so you'd need to go through Origin anyway. You could experiment though: open Origin and try to buy a Sims 4 pack. If it lets you—no need to pay, just get to the point where it asks for a credit card—then you should also be able to register a disc-bought pack.
By the way, if this is about pricing, Origin usually discounts expansion packs by 50% during its increasingly frequent Sims 4 sales. So you'd probably save more money by waiting for an Origin sale. Even if you did in fact need to buy the base game a second time, it's usually $5 during a sale, so the overall cost would likely still be cheaper.
——————————————————————————————————————————
I don't work for EA. I'm just trying to help fellow players with their games.
