I don't know if this is even a thing...
We currently create models and assemblies and we do this without including nut and bolts, for expediency.
Once the job goes ahead we add the sundries and use the bom's accordingly.
So my question is this:
Can you add an addition percentage to the Mass properties, to roughly estimate the final mass, for example:
Assembly Mass is 150 kg add to this 10% for nut and bolt weights, and have the result of mass come out as 165 kg?
Other than adding a dummy Part with that amount of Mass, I don't think there's any way to do that. Unless maybe a macro?