How can I add (kHz to MHz) high voltage signal to a high voltage DC offset?
The solution depends upon how quickly you need to vary the DC bias (if at all).

Just add a resistor in series with the DC source to the MEMS then a capacitor from the AC source to the MEMS.

The value of the resistor depends upon the leakage current in the MEMS, since they typically have very low leakage this could be 100k ohm.

The capacitor needs to have a reactance that is small relative to the resistor at the lowest frequency of operation. A value of 0.01uF would be appropriate for the frequencies you mention.