Strictly speaking, no, you cannot do this. The diameter of the pattern of the pins are different 20 to 30 amps. If no electrical inspectors are looking, yes, you can bend the pins and "make it work." I've done it, others have done it, but you shouldn't do it and it is a clear electrical code violation. Go to Lowes or Home Depot, but the right end, and stick it on there. You do still have the issue where the wire size is wrong, but if you have a brain and don't overload it, you will be OK. But, this too is still a clear electrical code violation.
I have a situation where a 208v/20A PDU (L6-20P) is supposedly hooked to a 208v/30A circuit (L6-30R). Before I order the correct PDU's and whip cords...sanity check...are connectors 'similar' enough that this is possible (with force) or am I going to find we've actually got L6-20R's on the provider side?
-- ~Randy