I don't know about your dahon, but all the recent ones are all built with the capability to attach a derailer hanger. The ones that come with the suntour derailer have an aluminum cover where the hanger would go, so you just need to get the dahon derailer hanger and replace it to use a regular derailer with it. it's only held in with one screw.
I don't know if it's a special part or just a regular part, but it's something thorusa doesn't stock so you'll probably have to go through dahon directly to get one.