You'll need the National Dex to access the Pal Park - meaning you need to have at least seen all 150 Pokemon in the Sinnoh dex - but then you're good to go. The Pal Park is where you need to be to transfer Pokemon over because you have to recatch them afterwards. Don't fret over it too much, though. The PokeBalls you're given to catch 'em are on par with Master Balls.