Your requirement to '…lose no data' is problematic, as there is no perfect 1:1 correlation of data between the Palm environment, and either the Address Book or iCal. You should therefore start by…
• backing up your events and tasks in iCal using the built-in Back up Database… option to do so
• backing up your contacts in the Address Book using the built-in Back up Address Book option
• activating the Palm Backup conduit if it is not already activated and insure that it is set to Backup
You will not be able to safely and successfully synchronize contacts, events or tasks with iSync and the iSync Palm Conduit under Mac OS X 10.5 or later. You will instead need to use the Missing Sync for Palm OS. Instructions are provided with that application to overwrite the contents of your handheld during the first synchronization - an important step to assure that you will not later have issues with data corruption. In your case, however, that's inconsistent with your requirement to preserve all existing records on your Treo, which may or may not already reside in either iCal or the Address Book.
So, your first synchronization with the Missing Sync for Palm OS should actually be configured to merge data between your device and your computer, not overwrite the information stored on your smartphone. But, assuming that procedure completes properly, you should next overwrite the contents of your Palm device contact, event and tasks application data files by performing a one-time, one-way event that others would have initially performed. The Missing Sync will subsequently automatically revert to bidirectional synchronization, and you will have effectively complied with that instruction.
Be sure that there no limitations set in your conduit settings which might adversely impact your historical data records before you perform your initial synchronization. Read the User Guide thoroughly, and you should have no issues at all.