I have a dyn menu, generated by one of mentioned plugins. I want to add a static anchor to the same page section among dyn generated menu items.
You could make a Weblink Resource Type and give that the #anchor-name
instead of a URL. In terms of displaying in the menu, this would allow that Resource to behave like any other Resource menu link, but on clicking it would fire whatever you put in the URL field.
Documentation here: https://docs.modx.com/revolution/2.x/making-sites-with-modx/structuring-your-site/resources/weblink