I have imported several different purchased subassemblies.  Many of these models come in as either surfaces or dumb solids. (however they are full of all kinds of components and air spaces I don't know about or need to model.  The manufacturers tell me what the weight is and if I need will provide locations for center of mass (COM).  In the case of a dumb solid I can give it a materail and then force the weight but I can't do that with the surfaces models and don't want or need to transform the surfaces to solids.

How can I assign a weight and COM to these models?  The "Mass Properties" is grayed out and while I can put a materail in, nothing else happens.