At the current development stage, it appears that the only way to add robots to the simulator is to described them visually using the OpenGL library functions. I believe it would be better if this could be done using a format that could be imported, as this would be more 'pluggable' like the rest of the simulator.
One format that would be particularly useful is X3D (http://en.wikipedia.org/wiki/X3D). This is nice as it describes the 3D bodies in XML, which is already supported by ARGoS through the tinyXML utility. It is also supported by the opensource modelling software Blender and would support most 3D software packages via conversion utilities.