You can add stuff inside head tag (between the head tags). Open your page in the editor and in the properties panel fold open the top dropdown menu. This menu contains all items on the page. Here the very first menu item is always "This Page" now you can click the button beneath it (the one after the word (Custom)) and choose the tab Scripts/Events and click on Header Code now you can insert something in between the head tags.
Easier way to achieve this is by double clicking somewhere on an empty part of you page and again choose the tab Scripts/Events.
My problem is to do that only once for a whole site (fortunatly not many pages) as a batch or something similar. From your answer I realize that probably is not possible.
Simply create an empty file called for example "include_scripts.dhe"; insert the code into this file then:
1) include this file into every page of your site
OR
2) in the export project options choose "Use a common footer for every page" and choose the file you've saved so when exporting Dynamic HTML Editor mixes the script code of this file with every page