Re: Will I be able to transfer all my Sims 4 DLCs to Steam from Origin???

Hi!  When you manage the downloads through Steam, it's actually the EA browser which pre-installs with The Sims 4 download that you're working with.  Often you need to minimize the game to access the EA app in the taskbar.  Once you see the EA app that loads with the Steam "play" selection, you have the option to manage your expansion packs if the emails are the same. 

What I mean by that is, be sure that when you log in to Steam that your Origin log in with expansions is the same email address.  Steam automatically logs in to the EA app with the Steam email on account, so if your EA and Origin log in email is different, they will not align, you will not see your expansions, nor will you have anything but the base Steam version of the game.

One thing I am uncertain of is if you can log in to the EA app loaded with Steam to change the log in credentials. I think this can be done through the EA app, by logging out of your EA account and logging in with the correct EA email, but that is beta level functional. The application (EA app) requires that the game be closed before proceeding which will close the EA app as well if done.  Try leaving the game on and running, select manage profile and go to an external browser to log in to the EA app through the browser.  Hopefully this will change the log in to the correct account, and then shut and restart the EA app, and The Sims 4 to see if the expansions are available.

I hope this helps.