How to add a new field to MB51?

 If the field is from MKPF or MSEG, then just fill checkboxes in 

SPRO >> IMG>>Materials Management>>Inventory Management and Physical Inventory>>Reporting>>Define Field Selection for Material Document List

If it is not, then 

Open the program RM07DOCS

1) Extend the structure

Before DATA: END OF list.  add the enhancement with your field.

 

2) Add the logic to fill in the beginning of  FORM output_list. 

 

3) Add the field to the field catalog in the end of, as for example:

  INSERT VALUE #( fieldname = 'MATKL' 

                  tabname = 'LIST' ref_tabname = 'MARA'  

                  ref_fieldname = 'MATKL' ) 

                  INTO TABLE  fc_flat.