You should be careful about the kind of USB <-> Paralel adapter. Many cheap adapters are just for printer and usually have 36 pins Centronics (like SCSI port). I do know there are an USB to Paralel port that has 25 pin D connector on it, try them and do try to buy only from the stores that will accept returns for full refund with no restocking fees.

Another possibility: paralel or serial adapter, try those as Mac serial are backward compatible with PC serial (although they don't have that one line CTS I think)

Another issue: if the program's DOS or Windows 3.x based, it won't work with USB at all as DOS never had an official USB driver. i know here are a few homebrewn drivers but there's no way of telling if they'd work.

If all else fails, get a cheap used PC laptop with parallel port and serial port and use program like zTerm on Mac to do a direct serial to serial file transfer.