if any site can do it it would be myfantasyleague.com
i dont see a direct way to have a rookie position, but what you could do is add a position your league doesnt use (i.e. Head Coach, O-Line, etc) and then manually change all the rookies to that position
a few drawbacks though would be that you couldn't start a rookie at RB though, although i guess you could make all spots as their position plus the rookie position (i.e. QB/HC, RB/HC) and manually monitor that nobody is starting a rookie QB at RB or anything like that
the other drawback would be that all the HC's would be on your WW and be eligible as well
so i think you could
do it on MFL, but it would be painful and would require some intense monitoring, but if you're league is honorable than maybe it would work
try posting in their forums as well...someone might have a better solutionhttp://forums.myfantasyleague.com/forums/index.php?
maybe even email them directly