Merge .ics file into existing iCal Calendar

Simple question: I have an .ics file that is generated by a combination of Automator and Applescript. Now I need to have that .ics merge, or update, an existing Calendar in iCal.

If I double click on it on the Finder, I get a window asking me which calendar I want to merge it with. How do I script the choice into Applescript?

The whole workflow is: Get info from the web, transform it into an iCal file, update the iCal Calendar. I’m missing the last step.