Thank you! From your hint about using ocean and some more searching, I was able to figure it out. When i export a 1dB compression plot to the calculator, I notice that the function compressionVRICurves is used to produce the plot. In the ocean reference manual I found that there is another function called compressionVRI which takes the same arguments and gives me just the value of the input power.
In case someone else requires this, the easy way to do this is to first plot the P1dB compression point curve the usual way (refer the to the tutorial mentioned in post #3. Export this curve to the calculator. You'll see the function compressionVRICurves being used. Just change that to compressionVRI. Copy that and use it where you want (in ADE XL's output's perhaps, like I did).