Add X-GOOGLE-CALENDAR properties to exported feeds

I believe that a similar feature was requested in the past, and I recall rejecting it for some reason.

Perhaps that reason no longer applies, though. I'll consider adding this, but I can't guarantee any kind of timeframe for when it will be available.

I'll add this to Date iCal 3.x. Eventually.

I need X-TAGS and X-COST which is often used by the famous All-in-One Event Calendar plugin at many Worpress calendars.

They also have: CONTACT, X-ALT-DESC, X-TICKETS-URL (not so important for me)

Wouldn´t it be possible that everyone can define additional source field names? Like at the CSV-Importer where I can define the name of every source´s field.

You should be able to add those to your importer by implementing hook_date_ical_mapping_sources_alter() in a custom module. The date_ical.php.api file has docs on how to do that. Assuming X-TAGS is a comma-separated value like the CATEGORIES field, you can use 'parseMultivalueProperty' as the date_ical_parse_handler.

I added that hook a few months back, and I realize now that it will enable users to do exactly what you need. I'm adding some additional documentation for the hook, and I'm going to mention it on the project page.