Can I add a LPT to usb converter and use it as port 2 (or any other port number)? My guess would be no.
If you will write your own plugin to interface that LPT port then you could do it.
Writting your own plugin means that you take care of reading and writting your LPT port and then you could do calls to the UCCNC via the plugin interface to control the FRO, SRO or to execute other function calls of the UCCNC.
However doing this requires programming knowledge and I also have to note that there is no direct LPT port access on 64bit Windows, so you could only interface an LPT port on 32bit Windows.
Can I use an arduino as an interface and use Modbus?
Yes, the UCCNC talks RTU, ASCII and TCP modbus protocols so you could interface a modbus device via serial port or ethernet.
And then you can write macros, macro loops or plugins to take actions in the UCCNC.
Or do I have to buy a UC300ETH?
That is definitely your easiest, plug and play option to go.