The short answer is yes, USFL players are eligible to play in the NFL. However, there are a few caveats that need to be considered.
First and foremost, the USFL was a professional football league that operated from 1983-1985. It was not affiliated with the NFL in any way, shape or form. As such, any player who played in the USFL is considered an unrestricted free agent by the NFL. That means that any team can sign them, without having to go through a bidding process or anything like that.
However, because the USFL was not as well established as the NFL and did not have nearly as much talent, most USFL players were never signed by an NFL team. In fact, of the roughly 1,600 players who played in the USFL during its three-year existence, only about 30 ever ended up playing in the NFL.
So while it is technically possible for USFL players to go to the NFL, it is very unlikely that it will ever happen.