Moving/merging a Google calendar to iCloud

I have a client with a proliferation of iCloud and Google Calendars,so that some of them overlap in function. For example, he's got both a "Work" calendar set up in iCloud and a "Work" calendar set up in Google Calendars. These have mostly different events. We want to simplify this by merging all overlapping calendars into single iCloud calendars, and also moving any unigue Google Calendars completely to iCloud. When we're finished we simply want a set of five or six iCloud calendars and no Google calendars.

I"ve tried to do this by exporting the particular Google calendar to a file via iCal's "Export" function, and then using iCal's "Import" function to load the events into the appropiate iCloud calendar. When I do this, I get the following error message pop up almost immediately:

Clicking on either of the choices does not seem to fix the issue, the message continues to pop up every few minutes.

Can anyone suggest a way to completely move and/or merge all the events in a Google calendar into an iCloud calendar?

Thanks!

BTW, we're running Mac OS10.8.2