Trouble connecting Thunderbolt duo to network via ethernet/thunderbolt connector
Short answer: You can't

The 2009 iMac is pre-Thunderbolt, and Thunderbolt is not backwards compatible as far as I am aware. 

From what I can see the DUO only has a Thuderbolt port, so you will be out of luck unless you can find a Thunderbolt to USB converter. I am not sure if those exist to be honest.

The only option I can suggest is to enable sharing between your iMac and your MacBook Pro and share the DUO of the MacBook Pro instead.