I have an iMac that stays home and a mobile MacBook Pro. I have a 1.5 TB Seagate external USB hard drive that is connected via USB hub to my iMac. This drive is the Time Machine for both computers. The USB Drive is always connected and running regular (hourly) backups from my iMac and (weekly?) full backups. My iMac is always on except when I explicitly put it to sleep every night. Every few days I remember to connect the USB drive to my MacBook Pro and let Time Machine do its thing. Every few days..

Time Machine appears to be working fine when connected to either Mac as I've restored a file or 2 recently off each Time Machine backup.
How can I tell Time Machine on my MacBook Pro to continue to back up to the same drive but now access it via the iMac that the drive is always connected to (without losing the previous backups)?
I've searched and only found how to 1)Initially begin using Time Machine over a network drive that was unsupported and 2)Move your backups to a bigger hard drive and keep on backing up via Time Machine without losing your previous backups.
I'm using latest Snow Leopard on both machines. Generic Wifi G Router (not Time Capsule or Airport Extreme w/external USB drive).