Please see How can I create a line that does not end at a node?
I am afraid, we do not accept contributions for yEd at this time.
You can, of course, create a user-defined palette section and publish the corresponding GraphML file for others to use. E.g. several such palette GraphML files have been published on GitHub:https://github.com/abesto/yed-aws-paletteshttps://github.com/gardar/yed-fa-palettes