I have a Pioneer DJ mixer model DJM-5000. Can I plug the X2u into the USB port of this mixer?
Unfortunately, that is not possible.
USB connections are a master-client type connection. Typically the computer is the master and the X2u is the client. Since the Pioneer mixer is also a client, it is not possible to connect the X2u to it. You can not connect two clients together.
A master has a USB type A connector (rectangular shaped like on your computer) while clients have a USB type B connector (more square shaped like on the X2u). Thus it is possible to identify a master by looking for a rectangular shaped type A connector.