If the threaded rod is 8-32 and less than 2 1/2" in length then what you have is a VEX standard screw with the head cut off.
You can also take the axle material and thread it. You won't get super great holding, but enough to use that as a spacer.
Anything other than that and it's not legal.
Official VEX rulings have a special place to ask
Offical VEX Q&A. CD's own beloved Karthik turns answers around there on a near daily basis.
As with FIRST, asking "legal" questions in the forums causes delays to you getting an official answer. (Someone has a cool signature file that shows the multiple day delay in asking here vs. asking in an official rules forum)
Good luck on your season.