Error while extending idoc type MBGMCR03

Hi experts,

I want to add few fields in segment E1BP2017_GM_SERIALNUMBER. For this I have created a custom segment with the fields which are needed to add. Now when I proceed with extending idoc in WE30 for the idoc type MBGMCR03, I am getting an error "Action is not possible for generated idoc types". Please help.

Thanks & Regards,

Linto Jose.