You can connect a USB to RS-232 serial
converter to the OpenCPN
computer. The OpenCPN manual describes how to configure a serial
port as an output port for NMEA repeater. There is a host of features where you can filter out certain NMEA sentences. But this may not be necessary as the VHF will ignore anything it doesn't care about. Set the BAUD rate for 4800. Tell OpenCPN which serial port number matches the USB adapter.
On the serial port adapter there will be a 9 pin male connector. Using a female 9-pin connector connect pin 3 (TXD) to the NMEA data wire on the VHF. Connect pin 5 (GND) to the GND wire on the VHF.
Please be aware that USB to serial adapters sometimes change their port number under Windows. If you keep the USB adapter plugged in all the time it should stay as the same port number but if you unplug it then the port number may change.