You cannot, in the base
jsprit API. What you have is a 3-dimensional bin-packing problem and they are not easy to solve (actually, harder for me because the literature is less complete than vehicle routing). As was raised very recently here, you would have to solve an inner combinatorial problem on each iteration of the outer vehicle routing algorithm. This is not sane ground for commercial application.
If you want to solve for vehicle routing, reduce the bin-packing problem down.