Event handling for standard save button in PO srm portal

Hello Gurus,

       I am into SRM 7.0 Ehp2 , Extended Classical Scenario. Now I am created two custom tabs (Custom Data and Retention) under header in po screen . Now I am facing few problems.

1. When I am opening a PO then all data's are in display mode except my ztab data.Still it is allowing to modify.

2. I have to code on SAVE button so that the  data entered into Custom Data and Retention gets saved in my ztable.But I can't find any way to enhance the standard SAVE button.Is there any BAPI/BADI or any class  where I can find the place to enhance the standard SAVE button of the PO screen.

Thanks

Gaurav Gautam