Can Grasshopper reference geometry from other products?

Grasshopper can reference any geometry in Rhino.  If you would like to reference a model created in Maya, AutoCAD, Microstation, etc., import the objects into Rhino.  Many Grasshopper definitions start by assigning existing geometry to a specific kind of component in Grasshopper called a Param. A good place to start is section "2_2_2_Components" in the Generative Algorithms book by Zubin M Khabazi.