The 8051 architecture doesn't dictate whether the hardware does DMA! Its just that most 8051's don't have DMA facilities. If you're using a 8051 softcore - then you can do what you want - it is SOFT! If the softcore is a 12 clock implementation you add some soft logic to allow memory access whilst the processor is executing an instruction or the smarter way is to modify the softcore bus logic to implement DMA. If the softcore control logic is microprogrammed then this should be fairly easy. At the instruction fetch phase, check for a DMA request and do the DMA cycle if required, otherwise check for interrupts and fetch/decode instruction.

If you're dealing with a softcore, one would think you have enough knowlege to tackle this issue yourself. It is not a trivial problem and the solution is not trivial. If you don't understand what I've written above, you've got a bit of learning to do.