Dear Jing,
you can use a volume distributed source if it is needed, just figure out geometry arrangement you need and possibilities of the BEAMPOS card. I just though from your scheme, that you need a beam, not volumetric source.
And yes it is possible to use several beam positions, but using SPOTPOS cards, which is similar to BEAMPOS. You can set sources programmatically using a source.f routine, but you will need to manage statistical weight of each source programmatically as well. See a relevant discussion here Multiple beams using source routine.
Cheers,
Illia