did you ever figure this out? because I have the exact same question.

I know how to add a Non-Steam game into the Steam list (its in the games menu drop down on the top.)

but I dont just want it added, I want it fully activated in steam and such.

let me know what you find out!

Trinity cannot be transferred if you bought it from somewhere other than steam, as it is not a steamworks game. As mentioned if you will get Rebellion its a rather minor point as it will build off of everything trinity has and is a steamworks game.