A USB to ethernet adapter is nothing more or less, but a network card with USB connection.
It is a card like a PCI ethernet card placed into a PC, but instead of PCI a USB port is used for the connection.
PCI cards are better because PCI has very high possible data throughput speed while USB can have a much lower connection speed, especially when talking about USB 1 and 2., however I've not seen a USB 3 network card so far. Maybe they exists, but probably not for the price range you looking for.
Hawever USB 1 and 2 speed is still enough for the UCCNC.
And to extend the single ethernet port you have on the computer you could also use a network router or switch, I mean to do that is also possible.