If you are lookingto do an assembly cut then you havea fewchoices:
1. use the Pub Geom method but create a surface feature instead of a solid feature. Copy the Pub in and then use it to "cut out" the desired void. You cannot copy in a solid and then use it for anything excepta positive feature.
2. Use the Master/Merge then cut out the void in assembly mode. Merge by reference if you want it to update.
3. You can copy surfs from parts as Bob sugestedbut make certain it all updates with a link into an assembly. No sense going to all the trouble to get one shot at it.