Imported Model Motor and how to deal with weight

I'm working on building a 3 lbs battlebot and I need to import a lot of motors and other electronics where there isn't a single material used to make them.  I need a way to make sure that I don't go over my weight limit.  How do I assign a density to the imported motor?  From what I have seen it looks like that may not be possible, at least not yet.  What is the next best way to design so that I don't exceed my weight requirement? 

It would be nice to be able to select a part and then assign a weight/mass to it without having to calculate the density.  For example, I mock up my motor that I purchased then assign a known weight to it.