Is there no way to create a custom shape from existing shapes?

Question

I am not able to create a compount shape from and existing shape.

Answer 1

Which shapes do you want to combine and to what effect? Can you upload a mock-up of desired compound shape?

That said, it is possible to import raster images and vector graphics into yEd (see How can I use my own node symbols in yEd? for additional information). Thus you could create the desired shape as e.g. a SVG document in Inkscape and then import said SVG document into yEd.