So far the Packers have signed about a dozen rookie free agents that were not drafted. The Packers are unlikely to sign any rookie free agent that has an impact or even makes the team, but that is how the Packers acquired RB Tony Fisher.
The only rookie free agent signed so far that I have heard of is southpaw QB Scott McBride. McBride is currently the 4th QB on the roster and due to receive a lot of practice and playing time this preseason, because I wouldn't expect Doug Pederson or Brett Favre to practice too much or play in that many preseason games. I would expect Craig Nall to receive the bulk of playing and practice time, with McBride playing and practicing as much as Pederson and Favre.
Many of the other rookie free agents didn't even start at college according to jsonline.com. Two other rookie free agents do merit some attention. LB Maurice Jones was a three-year starter at South Florida according to jsonline.com. The Packers have a number of candidates for backup linebackers, but none of them are locks to make the team. If Jones outperforms everyone short of the starting linebackers and plays well on special teams, he will make the team. The Packers only have two tight ends on the roster, so TE Keith Willis has a great opportunity to make the team. According to thehuddlereport.com, Willis is your typical athlete with all the size and skill (6'5", 260 lbs. 4.76 in the 40 yard dash) to play in the NFL, but he is an underachiever. TE David Martin did not improve last season, so a great preseason by Willis could make him the number two tight end.