You can buy Lion with the MBP. You should be able to copy the Lion Installer to the iMac. The question that I cannot answer is if the Installer will run on Leopard. Snow Leopard may be a requirement for the installer to function.
But if you do a web search you will find instructions for creating a startup disc from the Lion installer that will run from a DVD, a thumb drive and even an SD card.
But that raises another question. If you make a Time Machine backup of your iMac and then Install Lion, can Lion do a migration of your MT backup of Leopard? Or does it have to be a backup of Snow Leopard?