There are two different types of dongles - USB Dongle and Etherdongle. Both are able to be programmed for either DMX or Pixelnet.
If they are programmed for DMX, then you can go straight from the dongle to the Express.
If they are programmed for Pixelnet, then you would need to go thru a SS Hub to separate the DMX signal from the pixelnet, and connect to the Express. If you aren't running any smart strings at this time, I see no benefit to doing it this way, unless you are using a conductor. Only the Etherdongle will run the conductor, and the conductor currently is only able to be programmed for Pixelnet (not DMX).
So I guess my advice would be to get the Etherdongle, and program it for your needs.