Hi Mark Freestone - it sounds like what you want to have built is a Cloud Content app. It allows you to swap out specific elements of an email with content sourced from an external system, so people flowing through that step in the canvas can get personalized email content that is determined by that external system - could be all the same, or could be unique for each individual.
The technical specifics of how to build a Cloud Content app yourself are here:
of course we have partners building cloud content apps with other systems on the AppCloud, but if you are looking to integrate with a specific or home-grown system, then you can either build it yourself, or for much faster results, hire a partner to build it for you!
Egan