Are these parts within an assembly? Did you do this just to get the feature tree shorter?
Where it may cause problems is when some of the parts in the assy change position, you will have to change the corresponding hardware positions in the subassy.
If you were just trying to shorten the feature tree, you could add them to a folder instead which could be created in the main assy feature tree. The folder contents could then be collapsed.
The construction we're doing will not come pre-assemblied to the customer. The bolts and nuts, that I want to throw in that sub assembly, is the ones we'll throw in a plastic bag and then add that along with the rest of the components.
It's basicly to have a seprate bom structure, for the parts that will be added to the package.