Non-steam TC + Steam AP - is it possible? - Yes.

[mr.WHO]

Basically I have non-steam box version of Terran Conflict and I want to buy Albion Prelude on steam.

Will both games work together on steam?
I saw that there is a possibility to add non-steam game to Steam accout but I never used that feature - will it turn my non-steam Terran Conflict into Steam Terran Conflict? Also will the game added like that will be updated by Steam?

[CBJ]

If you have a non-Steam copy of X³: Terran Conflict then you can use this to register with Steam. This enables you to download the Steam version of the game and gives you access to Steam-specific features within it (see patch release notes for details)

By registering your game with Steam in this way it becomes a full Steam game, so yes you will receive updates and yes you will be able to buy X3AP to go with it. Registering your game is not the same as adding an ordinary non-Steam game to Steam; doing that would not give you the same benefits, so make sure you follow the instructions linked above. Note that you cannot register like this for earlier X-series games.

[mr.WHO]

Great!

I was afraid I'd have to dig out my TC box from my CD box (haven't used boxed game nor DVD in years since I switched to steam) - yet I see I can find the TC registration code here as i registered it