There are reasons no one has done it
At 5-feet-9, Nate Robinson is an NBA survivor and a talented, tough athlete. He was a decent college cornerback at Washington while playing both football and basketball for a year, but the odds are very much against him in this venture of trying to make an NFL team.
With a few rare exceptions, corners have lost the quickness to cover NFL receivers by age 31. Some who have spent the last 10 years playing in the NFL pick up enough tricks of the trade to still be able to do the job despite the loss of quickness. It might not be much of a loss, but just a little is enough to make a big difference at that level of elite athletes, especially if they don’t have the savvy attained through experience.
Of course, there is a genetic freak here and there who might be quicker at age 30 than at 20 but these outliers are rare … and usually it would not be an athlete, but a person who was a fat kid, not a college cornerback who went on to play in the NBA. That is, unless Robinson has access to and inclination to use the latest and greatest and untraceable little helper that the people who make the banned substance list aren’t even aware of yet, whatever it may be.
He could probably try to bulk up and play safety, but that’s unlikely. If he’s crazy enough and tough enough maybe he can make it as a special teams player, but he’d have to really, really want to do it and beat out a bunch of hungry kids who have been playing football the past 10 years. I also imagine he’d get some “special attention” from guys who would be eager to show that you don’t just waltz into the NFL from another sport.
With all that being said, it will be interesting to watch and I’d like to see him make it. I’m a fan of pioneers and have always admired Robinson’s ability to thrive in the NBA despite being about a foot shorter than the average player.