Sure, assign an approximation. Select the objects, go to Window>Rendering Editors>Mental Ray>Approximation Editor. Under the poly and sub division section hit "create", now the Mental Ray subdiv approx node should be in the drop down menu, if you want to add more models later just get back to the editor, choose it from the drop down and click "assign". If you don't think it's smooth enough, there is always that "edit" button and you can bump up the subdivisions.