Hi,
try this
implement interface IF_FPM_TRANSACTION in your custom WD component. This interface has method SAVE which gets triggered when SAVE button is pressed. Implement your logic to update your table here.
OR
we have PROCESS_EVENT method of interface IF_FPM_UI_BUILDING_BLOCK. you can try this as well. Use IO_EVENT to figure out the event.
thanks,
sankar.