Hi Zoran,
This isn't currently possible; the only way to create dashed lines is
to create a separate line string geometry for each 'dash'. You can
post a feature request to the KML issue tracker if you'd like, to:
http://code.google.com/p/kml-samples/issues/list
Since this is a general KML inadequacy and not specifically an Earth
API issue, it'd be best to file it as a feature request there.
- Roman