SpeedTree artist Sonia Piasecki gives you a quick overview of the Placing Meshes on Photogrammetry Meshes feature in SpeedTree v8.4.
This approach allows you to place imported meshes onto other imported meshes and have the ability to deform them.
Documentation: http://docs8.speedtree.com/modeler/doku.php?id=pg_using_branches
More info at http://www.speedtree.com