If I put multiple isolated DC/DC converters in series, can I add their isolation voltages?
If the insulation on one fails, the remaining one will cop 4,500 VDC which will definitely make it fail.

There's also no guarantee the voltage will spread evenly across them - the insulation on the devices might be measured in MΩ, but if one device is 1 MΩ and the other is 3 MΩ the voltages across each will be 1,125V and 3,375V and you are in for a bad day. 



It also bears mentioning that the 2,250 VDC one minute rating is a withstand rating, not an operating rating. You should not plan to apply this voltage to your device except to withstand the consequences of a malfunction.

Applying 2,000V on a regular basis is likely to cause cumulative damage. For example, "Megger" (hi-pot) testing a 440V cable using a 2,000 VDC test source is considered a good way to find cable insulation faults because it will blow through weak insulation and make minor faults into severe faults (which are easier to detect.)