You can download DLC from Bioware social network manually too.

[Volkira]

If you also register with the EA/Bioware site(Not origin site).
In you account details it list the games and all the dlc available for each game free,paid and unpaid.
Some are free and all you have to do is manually download the file then manually install it to your game.
All my mass effect 2 DLC were downloaded via the Bioware site manually.
Worth a look.

https://social.bioware.com/login.php?return_url=/user_entitlements.php

Edit:

Just to add you can also buy Bioware points from the site and purchase any DLC with them.
Once purchased it will show in your bioware account and you can manually download it and then install to your steam directory.

So you don't need to use Origin.

I have a hard copy installed and was able to use the key from it also on the Origin site to have it in my library.
Though my Deadspace 1 and 2 hard copy game keys weren't accepted at Origin.
So Rocketman544 see if your key will work at Origin since you want to go all digital.

[East]

thanks man

[rocketman544]

I had a question on this very subject--I originally bought a hard copy of ME2, but I'm trying to go all-digital now. Thanks for clearing up that question!