Xcopy can't do that in one step, but it could do it in two.
Copy all the files on the new machine to the flash drive
Copy the files from the flash to the laptop, which will overwrite any files on the laptop of the same name with the file from the new machine.
This will result in the laptop having all the files from the new machine, plus any files that did not get copied over. So what you have done create the end result that you want, only it's on the laptop
Copy everything off the laptop to the flash, and then copy it back to the new machine
A few extra steps, but is the flash is big enough to get everything at once, it will be a fairly quick operation.