Nope, tech stealing is totally impossible. I mean I guess in a very limited sense you might be able to kind of do that with abilities, but it would have to be limited to at most one or two ships per race, and you'd then have to "build" the ships not from your shipyards but from the ship that got the "blueprints". 

With what your suggesting probably would be difficult if not impossible. But your idea could be taken another way which can be done.  As in StarCraft-- you could in effect capture a targeted ship and control it for the duration of the game.  As for upgrading capabilities I doubt that.

No, I said absolutely nothing about capitalships...

Basically, you couldn't make anything generally do what you want. You could maybe make a ship that needs to "scan" a predetermined unit, say a hoshiko, and then after that the ship would be able to use another ability to spawn a hoshiko, but as you'd need a different ability for every ship, you at most could get 4 ships you could build. And not any for ships you scan, you'd need to determine which ships you want to be buidlable from the beginning. Also, the AI would never be able to use it right.

So yeah, basically you should just consider it not possible, the only way to do it is so lame it is not even worth the trouble.

Never hurts to ask....

The only thing that would probably be realistic to scan would be planets...having a ship scan a planet could give a permanent buff to any friendly ships that ever enter that well...that would work out a lot easier, though obviously it is not at all what you were originally looking for...