Do you mean players that will be in the 2008 Draft or players that went undrafted in 2007? If it's the latter, that's basically what current roster updates do.
Did you have any specific players in mind? The various current roster updates already include undrafted rookies currently on an NBA roster and in most cases, a couple who have been cut at some point during the season.
I added them when I did the NBA Live 07 update because I created every player who was drafted but as I did the NBA Live 08 update later, I only created players who were on an NBA roster (and a couple who were released during the first month of the season).