How do I copy a TimeMachine backup to an encrypted disk?
Apple has some instructions in this KB. There are some restrictions - Apple suggests that you'll need to format the new drive as Mac OS Extended with a GUID partition.

Short summary:


  Drag the folder "Backups.backupdb" on the current backup drive to the
  root level of the new backup drive.


Follow the other instructions in the KB, and you'll be able to use your new drive as the TimeMachine drive.