Create dopnet with rbd object (let's say "rbdobj") and solver, create pop object, pop solver and pop source, merge two branches together (it's better rbd be the first). In the pop source choose to create particles from DOP obj and type in "rbdobj".
then in Constant Birth Rate param of the pop source type something like:
100 * length( dopoption($DOPNET, "boxobj", "Position", "velx"), dopoption($DOPNET, "boxobj", "Position", "vely"), dopoption($DOPNET, "boxobj", "Position", "velz") )