If I install your cilent, in the steam folder, then will I still be able to play on the servers of original?

Yes. If you bought the games then yes. And no you won't be able to use the DLCs on Steam version(unless you bought them).

You better copy the game folder to somewhere else and then install IW5M on that copy, since we use some different fastfiles from Steam (localized_ui_mp.ff, for example)