I would like to add 2mm bigger than the fit bounding box size, to create my stock body. Is that possible to use any macro or add-in to assist?
You can try this macro: Create selectable 3D bounding box sketch using SOLIDWORKS API which will create a bounding box which is selectable. You than can create another sketch on top of it and add corresponding offsets. This bounding box is will maintain references once updated so your box should update accordingly.
You might also watch this free add-in: Geometry++. It doesn't currently have this option for box (only for cylindrical stock), but this has been a popular request so I'm planning to do some work on this add-in soon.
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