Besides importing SVG or PGN symbols to the palette you can actually create custom styled nodes by combining already exisitng nodes to a new node. You would do that by placing a new node on top of another node and invoke "convert node to label" on that node. This will transform the target node to an iconic label of the other node. By repeating this action you can create quite complicated shapes. Attached you will find some diagrams that make use of custom styled nodes
bio process: http://yed.yworks.com/support/qa/?qa=blob&qa_blobid=5112624030565966763
uml class: http://yed.yworks.com/support/qa/?qa=blob&qa_blobid=12154366786386398786
I have moved your question from Feature Requests to Help as this seems to be the more appropriate category.
It is not possible in yEd to create new node or edge types. You can, however, import images (JPG, PNG, GIF) or vector graphics (SVG) to be used as nodes. To do that, first create a new user defined palette section ("Edit" -> "Manage Palette", then click "New Section") and then import new symbols into your section (in the palette manager select the section and click "Import Symbols" or right-click on the new section in the palette component and choose "Import Symbols" from the context menu). See also 'Palette' Tool Window documentation in the yEd manual.