@ninipdi Sims 3 is old enough that it doesn't respond to account linking the way newer games do. But you should be able to register any Sims 3 content you buy in Steam within the EA App. Steam provides CD Keys that work in the App (sometimes) or on the old Sims 3 site (most of the time). If neither works, EA customer support should add the content to your EA account once you provide proof of ownership, which in this case would be as simple as a screenshot of your Steam library with the game and CD Keys showing.
However, I would suggest refunding the base game in Steam and going only with the EA App for now. Steam hasn't had a sale as good as EA's current sale in a very long time and may not for a while longer. Even if it does, prices will likely only be marginally cheaper than the current EA App prices.