This outlined process is not available in SAP.
Why would you need to repack?
Why don't you order in the right package right away?
If you would make use of the right process flow then you had already this subcontracting PO and would not need to create it after the GR of the ordered material.
you should have requirements for material B (repacked from Material A) e.g. from sales orders or just from reorder point planning.
This material B would need a special procurement key for subcontracting
you then get a requisition for B and if A is not available in your plant then you get even a requisition for material A that will be consumed in the subcontract order for B.
And in this standard process you could make use of auto PO creation via ME59N (even setup as batch job) to create your POs.