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.
They don't. Only games distributed and managed by Steam can make use of the Steam API.
Yes you can.
It is an addon that you buy from steam to use with other games to give them more features.
Yes
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Not US Imported games, I belive.
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.
This is possible, but only if the game already is available to buy from the Steam store. For example, if you have Half Life 1 from back when it was not on Steam, you can validate the CD key on Steam...and then play it without the disc through that. Games that Steam doesn't support however can not be translated over to Steam. Quite a few games are on Steam though, including EA recently adding a number of their games to it.
Yes, you can 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.
Britain was the main force in the invention and development of the steam engine and steam locomotives. The first steam locomotive to run in America, was imported from Britain, called the Stourbridge Lion.
You must buy them or use a hack to get them illegaly. I will not provide a link.