No, you can't use an LPT port. UCCNC does not support the parallel port, not even if it is a USB - LPT converter.
Yes, you can use Modbus but make sure it is REAL Modbus. UCCNC does have a Modbus plugin and that works well.
Buying a UC300ETH would be the simplest solution, but you need a license. I don't think you can "give back" the UC100 license and exchange to UC300ETH.