Re: How can I access pre owned expansion packs?

@GamerGrace2003  If the packs aren't listed on your account, either you never registered them, or you added them to a different account.  (Adding them to a store account also counts; the packs would be tied to your store login, which would also work as an Origin login.)  If you do have the content on a different account, you could ask EA to merge it into your existing account, at which point you'd be able to download the packs directly from Origin.  Keep in mind that the account that gets merged and disappears will lose any store content that it owns.

https://help.ea.com/en/contact-us/

The codes themselves are probably still on your previous computer.  Even if you uninstalled, the codes will likely remain unless you also manually cleaned the registry (Windows) or deleted the hidden Library files (macOS).  Here's how to find them:

https://sims3.crinrict.com/en/2013/09/how-to-find-your-serial-number.html

https://bluebellflora.com/finding-your-serial-key-using-terminal/

Once you have them, you can add them to your account and download them directly from your Origin library.