I am afraid, you are out of luck.
Exporting to JPG/PNG is similar to making a screenshot. If you open a Word document in Word and make a screenshot of the opened document, there is no way for you to edit the text in the screenshot with any Office application nor is it possible to convert the screenshot back into a Word document.
For PDF, the situation is very much the same - even though it is technically possible to edit the PDF document in dedicated PDF editor applications like Adobe Acrobat.
The one Office application that works even remotely similar to yEd is Visio. But the differences are still big enough that full-fledged Visio export is too much work to be feasible. Nor am I aware of any other tool that might be able to convert yEd's GraphML files into Visio files.
The best you can do is saving your diagrams as both GraphML and PNG. Share the PNG files with your co-workers. Whenever you need to change something, use yEd to edit the GraphML file, save, export to PNG once more, and share the updated PNG once again.
