Is it possible to change the physical properties of resources, such as the density of an object to be picked and placed by the robot, once it has been imported into a process?

 While the density and mass properties displayed in the Properties panel are not user editable, you can apply a material to each part. Once that is done, all of the mass properties will be automatically calculated and displayed on this panel.  To reset the material applied to the object, it is necessary to use the Apply Material function.  The CATIA-Real Time Rendering User's Guide as well as the Product Structure guide under (Modifying Component Properties) offer additional details.