Any trick to use my 2008 iMac (mini-DVI) as a secondary monitor for my MacBook Pro?
Yes - add a Mini DisplayPort port to your iMac. Should only take an engineer and a machine shop to accomplish.

In all seriousness, the mini-DVI port on your iMac is an output-only port, whereas the Thunderbolt/Mini DisplayPort ports also accept input. No amount of software, cables or dongles will change that.