How can I convert a very high positive DC voltage to negative DC voltage
Yeah seriously, the comment on your question is actually the right answer. Assuming the power supply outputs are isolated/floating (I'd be shocked if such a high voltage supply didn't have floating output -- and you would be too), then the easiest way is to connect the negative terminal of your supply to thing you want to energize to -20kV and your positive lead to whatever your reference is. The power-supply does what it always does and attempts to force 20kV between the positive and negative terminals. Voltage is always a differential measurement, and with the floating the supply you can choose your reference to be whatever you like.