It doesn't matter using the document type NB or UB. you can also use stock transfer between plants by using document type NB. You just need to set up some settings for this document type and you need to use item category U. SAP have just defaulted the document type UB for stock transfer.
By the way, I just assume why you need to adopt the STO facility, while you just can use 301 movement type for plant to plant transfer (303-305 for two step) and 311 movement type for Sloc to Sloc transfer (313-315 for two step).