You can't back up WHS to a NAS drive if you mean backing up the server. That needs to be done on an external drive physically connected to the Server. However, you can back up data in shares to your NAS. I do something similar using SyncToy on a client
but you can use many other programs such as SynBback, Robocopy etc.
Phil P.S. If you find my comment helpful or if it answers your question, please mark it as such.