Passing wire to another panel
I have 2 150amp panels tied together with a 2" PVC nipple (less than 3" long). The first panel is the main panel, and the second is a sub panel with a 100 amp 2 pole breaker in the main panel. I have fed the sub panel with #2 copper for the 2 hots and the neutral, and a #6 for the ground bus bar in the sub panel. All these wires pass between the two panels via the 2" PVC nipple.
Can I use the remaining space in the 2" conduit to pass circuit conductors through from the main panel into the subpanel to move some circuits into the sub panel? This would be rather than re-routing the wire on the outside of the panel over to the sub panel, covering up the open holes on the main panel, re-stapling the wire, etc. I'd only be looking to move a couple circuits, maybe 3 or 4 total.
Mainly I need to create a little space to connect another subpanel off the main panel to feed the attached garage at some point, and to connect some additional circuits for my server room. I can physically move the cables coming into the top of the panel over to the sub panel, but I'd rather not if I can avoid it.