I LOVE YOU STARDOCK. I finally get the money to get Rebellion and I was almost sick at the idea of having to pay Gamestop a single penny since I bought the original game and expansions pre-Trinity off impulse, and was actually worried I wouldn't be able to play Rebellion if I just bought it from your store. This post means I can finally uninstall that abomination from my computer and never look back. While it may be unfortunate that I cant bring my pre-Trinity copies over to Steam, (don't suppose you would be willing to give me a Trinity key I could use there since I own the game and the expansions already?) the fact I cant play with people who don't have rebellion is not going to be an issue, since everyone I would play with already got it a while ago.
Log in to your stardock account. Check your downloads section. If you have a registered Sins Trinity key, it should be there so you can download it. And the key should be there next to the download. This no guarantee, but there's a good chance it's there so you shouldn't have to buy it again. That's where I found mine when I no longer wanted to use impulse.
Your login should be same as here. Once logged in, click on "My Downloads". If your Trinity is registered, they should be there in the games section. You will still need to activate it via stardock, but you won't need to use steam nor impulse to install nor play trinity.
As I said, there is no guarantee. If all your downloads were bought and downloaded from impulse, it will probably still show the keys, but might require you to use impulse to download and install. But it's worth checking in any case.