Is it possible to switch relay above the power spec?
Yes, you can do this.  The important thing is that you aren't switching 6kW.  When the relay switches the voltage is zero, so the wattage is zero.  And when power is applied, the relay sees either 3 amps and zero volts, or 2000V and zero amps, all within spec.

However, you need to be very sure the relay doesn't switch under power!  The first switch event under power may be the relay's last, and the 6kW will make a spectacular failure.

Edit:  In response to comments, the relay's safe operating area looks like this:



The area is bounded by I=3A, V=3500V, and VI=200W.