How to migrate several usb Time Machine backups to a Time Capsule?
Finally, I decided to give a try by myself. I had no particular problem, I just followed the well known procedure of backup to Time Capsule, interrupt, and restore the USB drive to the Time Capsule sparsebundle related to the machine.

All my previous backup, have ben recognized.
However, for 500GB of data I had to wait for two days to be completed. 

Apparently, by matching the two .sparsebundle size, the data seemed to be transferred in a reasonable time from 4 to 7 hours. For the rest, it just seems that disk utility stopped doing nothing for the rest of the time. 

Even the USB drive went in stand by mode for almost two days. Then, after two days, it just stop informing that the restore went fine.