Trying to activate Mass Effect on Origin but will not accept Steam CD Key.

The first Mass Effect is from a time when EA Download Manager (since re-branded as Origin) did not always accept product codes from game copies bought outside of the EA store. That's why the original Mass Effect will not always be accepted by Origin.

You can, however, contact support and they can assign it to your account. Several years ago I did the same thing with my original DVD copy of the game. I went through live chat, they had me make a Pogo account for some reason, but in the end it all worked out.

http://help.ea.com/contact-us