KAlarm accesses remote calendars only using remote file access protocols, and it doesn't implement other protocols.  Because it is primarily a personal reminder application, as opposed to a group one, other remote protocols seem unlikely to be required by many users. KAlarm used to use Akonadi, the KDE PIM data interface, to access calendar data, and this could potentially have been used for caldav access, but it caused so many bugs that it was eventually abandoned in favour of simple file access. This has had the extra benefit of keeping things simpler. So unfortunately for you, it is unlikely that remote protocols such as caldav will be implemented.

That's a pity because kalarm is wonderfully useful and I'm addicted to it but when I'm not at my PC I lose all the informations and reminders and I'm lost :-(