Unfortunately IX was never made for the PC originally so it's unlikely to appear on Steam, unless Square Enix have been working on it secretly but they don't exactly have a reputation for giving their fans what they want in recent years.

That said I through VI is entirely possible, it's a case of whether or not Valve and Square can work out copyright issues and so on because they'd probably have to be emulated rather than ported to PC like VII and VIII were. We'll see in due time no doubt.