Adding new screen to standard transaction ?

Dear ABAP Experts ,

                     I have a requirement that when the user select payment term value as 'LC' (Letter of Credit ) then they wants to generate new screen in ME21N transaction. In that screen there are many fields (which are field by manually). In that screen they want SAVE button. When the user press SAVE button then all the data(New Screen fields) will store in Custom(Z or Y)table.

   If the user will not select LC the process is normal only.

This is possible to implement ?  How can i do this ?

Help me the steps required to create new screen for the above mentioned scenario.

Thanks & Regards

K Chinna..