Is there a way to plug a NEMA 6-20P plug into a NEMA 10-30R receptacle?
You will not find a legal or listed "adapter" for this. 

A 6-20 is a straight 240V grounding 20A device.
A 10-30 is a 120/240V non-grounding 30A device, where the grounding function is provided via the neutral. This is only allowed in older range and dryer circuits, so any adapter would not be legal to use on any other circuit.