Boost a HV power supply from +1250 V to +1500 V
Get hold of a properly isolated 1:1 AC power transformer. Rectify and smooth the output (~330V DC). Put the rectified output in series with the HV power supply output. Job done.

This does rely on proper isolation between primary and secondary and if in doubt wind your own.

An alternative might be to buy some dc-to-dc converters that are properly isolating and float the outputs up.

If you are worried about floating the above outputs up you could float the actual HV output up by 250V DC maybe. It looks like you could put a dc-to-dc converter on the LV side of the device so it then becomes an isolated output. Then, raise HV ground to 250 VDC.

Alternatively just buy the next model up - the C15 model - it does 1.5 kV.