ABAP WD, ALV, assign event to dropdown

hi fellow ABAPers 😊

i am enhancing a standard ABAP Web Dynpro (Timesheet, Component HRESS_C_CATS) which uses an ALV control. The ALV contains a dropdown field (by key) and I would ideally like to have an event triggerred BEFORE the list is displayed to the user. This is because I wish to modify the contents

Does anyone know if this is possible, and if so, provide some code examples (or links to code).

Points will of course be rewarded to those answers that provide the solution (or strongly point the way).

Many thanks in advance for your help ........

Jon