One way would be to save the sub-assembly as a part (or start with the bolt, then go to Insert > Part to add the nuts and washers), then use that in the main assembly instead of the sub-assembly.
Hi. hope i underdtand you right.
sometimes we use Glenn idea but it's require a lot of work,
1. you can use configuration on the sub assembly.
and let the "BOM" use the file name so all config' run one the same number.
2. cheat the system
by taking them out of the "bom" (under properties of the sub assy)
and rewrite the values