You don't really need N2K to USB. 0183 coming out of a plotter will usually give you all the data
OpenCPN can use.
Tell us more about your setup and we might find other ways for you to do it as well. If you have a plotter with
WiFi that would be the easiest way to do it.
I have, in years of using
OpenCPN, done it in different ways:
1. 0183 from the 0183 talker port on the back of my
B&G Zeus, to a simple and
cheap RS422 to USB adapter (but no
AIS data that way)
2.
Wifi from a GoFree module.
3. Ethernet from a spare port on the back of the Zeus.
4. Just
GPS data from the USB port on my Standard Horizon HX870 handheld
VHF.
It all works; the Ethernet has the added advantage that you can get
radar data. And there are other ways.
If you did for some reason want to get the data off your N2K
network, then the best way is with the Actisense NGW, which costs about €180 hereabouts.
There are various other choices;
Google is your friend. E.g.:
https://www.yachtd.com/purchase/?ydnu
https://digitalyacht.com.au/product/ikonvert-usb/
ShipModul Maritieme Elektronica
All roughly the same price except this one:
https://www.maretron.com/products/usb100.php
But you might think about using a converter with Wifi so you can connect tablets as well, and not just a fixed computer. The price is roughly the same.
NOTE: Some NMEA2000 adapters, like the Actisense NGT, do NOT convert the N2K PGN's to 0183. OpenCPN CANNOT talk to such devices. Don't ask me how I know!