Move existing USB Time Machine backup to OS X Server Time Machine
You will need to enable and configure the Time Machine service on the Server as per this set of instructions. Pay attention to any size limits you may want to enforce for the Time Machine backups when creating the destination location.

Turn off Time Machine in System Preferences. Copy your existing Time Machine backup structure to the new location. Open Terminal and use tmutil to re-associate your backups on the new location. Use the instructions here for doing this with tmutil. Restart the Time Machine service and you should be set to go.