Is it safe to change a nema 6-30plug to a nema 14-30 plug?

You CAN build your own adapter cord but it would be frowned upon. The best bet would be to change the plug on your kiln. Wire in the two hot wires and the ground and you would be ready to fire it up. I don't believe you will find an adapter to fit these two plugs, but if you can find the mates to the two, you could build your own adapter.