If you are asking if it is possible to have your Palm handheld data overwrite the iCal file, the answer is no, as this process is not supported by the Sync Services framework.
If you are asking if it is possible to merge the data on your device with the data stored in iCal, that can be done with iSync alone.
If you want more extensive support of later Palm devices than that offered by iSync, then you should take a look at the Missing Sync for Palm OS, and its support for iCal through the provided Mark/Space Events and Mark/Space Tasks conduits. Note that you are expected to overwrite the contents of your handheld with the data stored in iCal during your initial synchronization, and the failure to do so can create all sorts of issues during subsequent synchronization events. This is a problem if, as outlined in the first paragraph, the data you want to move into iCal resides only on your handheld.
It is possible to export data from the Palm Desktop as a single vCal file, then simply double-click on that exported file to import those records into iCal, at which point you can overwrite the contents of your handheld with the records stored in iCal, as you won't lose records by doing so. You may lose some information presently on your handheld if the iSync Palm Conduit does not support mapping of it. Event categories are one example. To avoid this issue, you should purchase and install the Missing Sync for Palm OS.