Snap Footprint polygon to building multipatch

I have a set of multipatches that are composed of roofs and walls.

 

 

I have derived a footprint polygon which I want to snap to the base of the building multipatches to make them "watertight". The base of each wall is sloping to some varying degree (ie wall ZMin does not necessarily equal the multipatch ZMin).

 

 

I can do this in ArcObjects but I can't figure out how to do it in FME (2013 SP2).

 

 

Does anyone have any ideas?

 

 

Thanks

 

AlanK