Not sure which bits your meaning on 3040 to fill but I've got the slightly smaller 3020 here and the only part of the machine that could be filled is the bed extrusion.!

No way will you get concrete or Epoxy granite down the small extrusion holes due to the aggregate size, Epoxy alone won't had much resonance dampening or weight.

The thing you could and I would do is cap seal the extrusion at one end then pour kiln dryed sand into then cap and seal the other end. This way you can empty the extrusion if need be for moving about.!! . . . Far better and really simple has the extrusion end is pre drilled ready for tapping for bolts to hold the caps.
Sand is a brilliant resonance dampener and heavy.!!