It's completely possible if you wanna rebuild the entire game from scratch. If you were to go to unity spend who knows how long, you could totally "remaster" it for vita. However, you'll end up spending tons of time and effort for one game at a time.
On the series s/x you can use a external HD but any next gen games must be on your internal ssd, or you can buy a ssd expansion card, but pricey. Any xbone, 360, or og Xbox games can be played from external tho.