Using Existing Time Machine Backup over Time Capsule
You are correct. Time machine makes a sparse disk bundle to hold backups on Time Capsule and instead uses a folder for attached hard drives.

Due to the security settings on the backup files, it's not as easy or reliable, but here's how you could start:


Get a first backup or two on the TC to set up the sparse bundle
Disable Time Machine
Mount the Bundle and disable the security that keeps you from writing to it
Delete the current backups since they likely won't mesh correctly with the old ones. (I'm not 100% sure this is needed - but I would do it)
Copy in the old history
Re-enable the security and ensure file permissions on the Time Capsule files are correct.
Enable Time Machine and start a backup to see how it goes. 


See this macworld article for a version of the commands to disable and then re-enable the protection on the sparse bundle disk image. I haven't done this as I would rather not muck with the internals of Time Machine. I know I only have to connect the old hard drive to my mac to see all my old history. The risk of using old incorrect information isn't worth it to me - but that doesn't mean you don't have a good reason to move the files over. If I were to do this, I'd consult with a Apple technician as they gave me specific instructions on 10.5 when I had a MLB swap on how to migrate the old Time Capsule backups to work with the "new" mac. Do have a look at the end of the Time Machine lesson in Mac 101 There's a nice tip on option click of the Time Machine menu bar icon to view files on older volumes than the current one.

Good Luck