Actually I'm pretty sure A league clubs can sign players from other Clubs NYL sides, as long as its for the first team.
Could be wrong, just remember hearing it somewhere.
Edit: RR found it for me. NYL players are basically fair game and not tied down to their existing club. If an A league club offers a senior contract to an NYL player of another team, the existing team has 7 days to match or better that offer - but even if they do, the player can turn it down and move to the other club.
So they're basically free to move wherever.