It appears that xgml to png conversion represents export of visualized graphs saved in XGML format to a standard PNG graphics file, something that should be doable with any tool that can open and process .xgml files.
Usually dedicated preview export is available, but if it's not in your tool, you can always use screen capturing software (or just PrintScreen) to get the picture.
Tools like yEd are most likely your best option how to achieve this, as we do not have information about any dedicated converter for this procedure.