@addiedolgner There's no way to transfer DLC to Steam directly, but you can use your EA App-owned DLC in a Steam install. First, uninstall Sims 4 through the EA App if you haven't already. Install the game through Steam as well as any packs you own there. Launch the game once to make sure it works; if not, stop here and let me know.
Otherwise, sign into the EA App and download your DLC: just the packs, not a second copy of the base game. The data should go into the folder for the Steam install, and Steam should then load all your DLC.
If this doesn't work, please run searches for two or three of the packs that aren't loading. Open a File Explorer window, click This PC in the left panel, and enter the folder name for one of the missing packs in the search field in the upper-right corner. This is the full list of packs:
https://sims4.crinrict.com/eng/extras-2/available-packs-dlc/
For example, if you're looking for Seasons, you'd search for EP05 . Wait for the search to finish, then make sure the full file path is showing under each result. If it's not, expand the window sideways as necessary. Take a screenshot using Windows key-shift-S and attach it to a reply here. Then repeat for one or two more packs.