Unable to extend ARTMAS05

Hi,

As per one of requirements we have added 4 custom feilds to mara, Now we need to send this via outbound idoc . To do so i was trying to extend

artmas05 but it gives error that "Action is not possible for generated

idoctypes". Can you please let me know that why artmas05 doesnot supportextesnion and how do i transfer my custom feilds. I dont want to copy

artmas05 to zartmas05.