If there was any evidence that this was at all common in the long history of these leagues, then yeah, maybe. But it's not.
The reality we all know is that hardly any one watches these games and at some point the broadcasters don't want to eat the losses incurred by producing them and the plug gets pulled. Until that inevitability, every NFL team is free to sign a guy from one of these leagues. It's so rare that they do--this means that the NFL has already done this math and they know there is little value in supporting those rosters for the very rare "call-up".
Also, the NFL draft is a month after the UFL season starts.