Can I hold PGI using an User Exit?

Can I hold PGI using an User Exit?
We want to ensure that the tracking number is entered into the delivery doucment before PGI happens.Is there a way to stop PGI unless the information has come in.Currently the delivery document is ready for PGI,once the transfer order is confirmed i.e pick status = c.What we want is that even though the pick status is "C",do not PGI unless tracking number is entered.What I am looking for is :We will create a z field called "ready for PGI" in delivery document.By default it will be unchecked meaning it is not ready.When the interface enters the tracking number it will check-mark the z field making it ready for PGI.The PGI program should check if this field is marked,in addition to all the other checks it performs,before carrying out the PGI.Which User Exit can I use?Subbu