I would try this first
If it is showing up as part of the mesh in the scan then it has already joined just Hubitat didn't add it.
I had it happen once and I am overly paranoid worried that it would not work correctly.
So I actually set the Xbee settings for an end device (see first post) and then put hub in pairing mode and added the Xbee as an end device then removed it and reset the settings back to a router and readded back to Hub.
But that was a long time ago and it was a pain. I think just using jrfarrrar suggestion and leaving it or then deleting it then adding it again would be faster. You will know that it worked for removing the device if after you create a device and add the address's then delete the device and you then do a XTCU scan (before trying to add the Xbee again).
The scan should not see anything other than the Xbee.
Then put the hub in pairing mode and add the Xbee again.
(to be clear you should not need to change the Xbee settings to an end device if you do it this way)
