Automatically adding a new PR into existing PO

Hi Sap MM Gurus,

I have a requirement, where i need to convert the new PR into a PO which is already existing. or simply i need to add PR into already existing PO and this should be done manually without any manual intervention.

We have created a new field in PR screen to add existing PO number for which PR needs to be added. Can somebody suggest if there any standard functionality available or any note or coding that will help us to achieve this result.

Thanks,

Manjunath K