After looking and trying for hours I couldnt manage to solve the problem myself.
I got many KML files, they include routes, and could successfully set them up with Comprehensive google map plugin by alexanderzagniotov.
Now I want to add the plugin's markers to this KML routes, so that they are loaded together in one map.
The plugin seems not to support this feature by default. Adding markers to a map with KML file in the shortcode builder, the resultant map only shows the KML datas.
Many thanks in advance for your suggestions!