Can hot swappable power supplies be used in series to provide increased voltage?
ATX or other PC power supplies cannot be used in series.
Output ground is connected to Earth via a low impedance or direct short.

You must use laboratory power supplies rated for floating operation with higher common mode voltage.
Not all of them are! Read the manual.

You know they are capable of floating operation when it is written around the connectors.
For example on this Rohde and Schwarz HMC8043 maximum common mode is 250 Vdc: