I've no idea how you'd make new custom shapes in PSE or PS, but there are lots of free extra shapes around the web so, logically, there must be a way to make them or there wouldn't be anywhere on the web to get them except through Adobe.
There's a big collection to be found at Sue Chastain's site
http://graphicssoft.about.com/od/photoshopdownloads/ss/shapes1.htm.
So there's just about got to be what you want there. Don't forget that each .csh file is actually a collection of shapes, and the files go into the Presets/Custom Shapes folder.
If you go to the shapes box and click on the double arrowhead >> at the top you'll find a lot of ready-made custom shapes and, as they can all be transformed/sized/coloured I can't see a need for that many more, unless somebody required a very specialized collection, like geometric shapes, animals and fish, maybe sports. (ooo there's a good idea!)