Yes you can, you import and add non-steam games to your Steam library.
From within the library you can launch and run any games, although 'Steam achievements' will only be available for Steam enabled games.
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Yes you can.
Yes, some. Not all of them can be activated on Steam. See the related links for a list of the games that can be activated on Steam and for the instructions on how to activate a game on Steam.
yes.if you use valves steam community you can use the steam cloud feature to put games from windows to apple mac
You can't.
No, but there is a steam function to add non steam games on to your library.