BAPI_OUTB_DELIVERY_CONFIRM_DEC could be used for decentralized system. I am not sure about this, but it seems that the serialization does not work when you confirm from a decentralized system
Perhaps, you might want to check this
http://help.sap.com/saphelp_470/helpdata/en/34/a9eb3c1875bd48e10000000a114084/content.htm
So what I would suggest is to check with some functional consultant if SAP can generate the serial number by tweaking some settings.
Another option would be to get the serial number generated, using some BAPI / RFC like DPWTY_SERIAL_NUMBER_EQUI_GET/ BAPI_EQMT_DETAIL, for the line items and then populate them in parameter ITEM_SERIAL_NO of BAPI BAPI_INB_DELIVERY_CONFIRM_DEC