We need a little clarification here before anything else.
The RJ45 is a connector type and not a cable. A RJ45 connector has 8 positions and is commonly used for LAN applications using a CAT 5 or CAT 6 cable. The actual pin out can be configured for a patch cable or for example a crossover cable in LAN apps. Of the 8 lines only 4 are actually used in LAN applications. There are 4 data lines and no power lines within the LAN configuration.
This is a good read
on that note. Just remember there is no RJ45 cable. RJ45 is a connector type.
USB is just that it is a Universal Serial Bus and only has 4 lines used. However those 4 lines include power of 5 Volts and a common and only 2 data lines. In this case the connector used is called a USB connector.
Any connector can be used in a custom application depending on if the connector will do the job required of it.
explains many of the LAN protocols in use.
explains USB protocol.
I have seen a build where a guy used a eithernet switch for a itx build and made custom rj45-usb cables and then used the eithernet ports for usb ports.
so i would say it is possible.
The key word there is custom. Yes, that can be done.