Floating power supply in series with HV earth referenced source

[wolf32d]

In our lab we have a good old Agilent E3631A. Its 6V floating output is connected in series with another mains earth referenced power supply.
In other words the second power supply lifts the negative socket of the E3631A of a voltage V_lift.

Here's the question:
what is the maximum safe value of V_lift? I cannot find any data about the output-to-earth ground insulation 

[David Hess]

Page 161 of Agilent E3631A manual:

Isolation
The 0-6V supply is isolated from the ±25V supply up to ±240 Vdc. Maximum
isolation voltage from any terminal to chassis ground ±240 Vdc.