Pokemon Bank allows you to transfer Pokemon from any game (compatible with it) to the Bank, and vice versa. But I think its compatibility is only for the 3DS games? I don't know, if you bought X/Y Black/White Alpha/Omega maybe between the 3 but I'm still pretty sure there were some Pokemon you could only get through events (certain legendaries).
Someone else might have more information on it here... I can't be certain
Edit: According to an article, Its possible in Alpha/Omega to catch them all, Minus the "Mythical" ones, I.E Mew, Jirachi, and a few others who are only given out during special events (or through trading). Though I don't know how accurate it is.