got it... the file define_about_us.php did exist but was placed in includes/languages/german/html_includes/MY_TEMPLATE/define_about_us.php
this was working in the frontend as the override system successfully displayed the content of this defined page.
i copied the file to
includes/languages/german/html_includes/define_about_us.php and now i can select it in define pages editor.
i just compared this structure to about_us mod and noticed that the mod uses the same file twice
includes/languages/german/html_includes/MY_TEMPLATE/define_about_us.php
includes/languages/german/html_includes/define_about_us.php
but i do not understand this redundancy as both files have the same content. by using the define pages editor i will edit includes/languages/german/html_includes/define_about_us.php
but it will always be overridden by includes/languages/german/html_includes/MY_TEMPLATE/define_about_us.php
on one hand i want to use the override system but on the other hand i want to make use of the page editor. but will the editor affect the override file also or is a change by define page editor senseless because i need to edit the override file by hand?



