Насчет USER EXIT/BADI не знаю, по понятным причинам сейчас нет доступа к САПе. Но на крайняк можно все field exit'ами сделать.

To create field exits, choose Utilities in the ABAP Workbench. Choose Enhancements and then Project management to edit field exits and to implement customer exits. Do not create field exits directly from the Function Builder.
Choose Goto -> Global enhancements -> Field exits to start the transaction for maintaining field exits. To create a new enhancement, use the menu path Text Enhancements -> Create.
Enter the name of the data element to which your screen field refers in the modal dialog box. The Function Builder is started with a special naming convention and interface options. The system specifies the name of the field exit. Do not change this name. Create the function module in a customer function group.
The function module must be assigned to an existing customer function group.
The function module interface is fixed and cannot be changed. The function module has an import parameter INPUT and export parameter OUTPUT. The contents of the screen field are stored in parameter INPUT. The contents of OUTPUT are returned in the screen field when you leave the function module.