Automatic Purchase order - Assign Document type

Dear Gurus,

Basically, PO (Stock items) is generated in NB Document type with 45XXXXX series. Now business required to implement the Automatic PO generation. Also whatever the PO generated through ME59 should have different number range (say 90XXXXXX) to differeniate these PO are generate Automatically through Outline Agreement.

In order to have different number, i need to create new PO document type, BUt how to configure only for AUtomatic PO generation through ME59 or ME59N

Please advice

Regards

RS