XxgagelaxX For a Windows install, which you must have if you're playing Sims 4 through Steam, you can combine packs you own through Steam, the EA App, and Epic. For Epic, you'd need to use it to launch Sims 4 once, and then the Epic-owned packs would show up in your EA App game library and be usable there.
Since you own content in Steam, you'll need to have a Steam install, since this content cannot be used in another type of install. If you only buy through the EA App, you don't need to do anything special—just make sure the EA account that buys the content is the same one linked to your Steam account.
Open the App, click Sims 4 > Manage > Manage add-ons, and your new content should be available to download. Don't install a second copy of the base game though, or else the EA App will install your packs into that copy and not the Steam version you'll actually be using to play.
If you buy content through Epic, you'll need to install the game through Epic and launch it once, then go back to your Steam install and download the Epic-owned content through the EA App as above.
