if you don't already have a NAND folder to be used for "emuNAND channel" on your USB, you can safely copy or move your existing folder from SD to USB.
Then go to USBLoaderGX > settings > user's paths > EmuNAND Channel : browse to your folder on USB and select it.
example :
before:
on your SD : /nand/my_dump/
on USBGX : EmuNAND channel folder : SD:\nand\my_folder\
after:
on your SD : no more folder
on your USB : /nand/my_dump/
on USBGX : EmuNAND channel folder : USB1:\nand\my_folder\
If you already have a NAND dump on your USB, you should not merge the SD dump into it, or it will create issues.
instead, make two different folders, and switch to the one you want to list/use when needed by editing the "emuNAND channel folder" option.
be sure you are editing the "EmuNAND Channel" and not the "EmuNAND Save" option, which is a different option affecting Wii disc based games only.