How do I move a USB Time Machine backup to a Time Capsule? Is there a safe way to move a Time Machine backup on an external USB hard drive to a Time Capsule?
Yes, it's straightforward to do it, even though it would take quite sometime to complete depending on the size of your current backups. Follow the detailed procedure at Copy  Local  backups to be used on a  NETWORK.

Here's a summary of what's required and the steps.

The backups go into a sparsebundle on a Time Capsule. So you need to create that sparsebundle on the Time Capsule, mount it in Finder and then copy over everything from your USB drive into the mounted volume.

Here are the basic steps with some additional information:


Configure Time Machine to back up to the Time Capsule and start a backup. This will make it create the sparsebundle.
Cancel the backup once the copying starts (as seen in the progress window).
Mount the Time Machine Backups sparsebundle in Finder by double-clicking it (or using Cmd+Down Arrow)
Delete the Backups.backupdb folder inside this volume (you'll be replacing it with the backup you already have).
Open your current Time Machine backup USB drive in Finder and copy the Backups.backupdb folder from it to the Time Machine Backups partition (mounted from the Time Capsule).
Enter your administrator password and accept the prompts (this may happen a couple of times).