Is it possible to add some own code to the tracking code from the plugin?

Hey Juergen! Thanks for posting your question!

Unfortunately the traking code is not editable yet, but if you wish to add your own script, you can add it to /wd-google-analytics/admin/tracking.php file of your website.

Please give this a try and let me know in case you will need further assistance.
Thank you! Have a great day!