Importing parameter of this BADI method is an ABAP Object (Class/interface). If you need to update any values, you can respective methods...
Below is the sample code from the link which I shared...It calls GET_DATA method for getting the current line item data and changes the work area and sets back to buffer using SET_DATA method.....
CLEAR: LS_ITEM_DATA.
LS_ITEM_DATA = IM_ITEM->GET_DATA( ).
MOVELS_ITEM_DATA-ERNAM TOLS_ITEM_DATA-AFNAM.
**Get source of supply details
CLEAR: LS_SOURCE.
LC_SOURCE = IM_ITEM->GET_SOURCE_OF_SUPPLY( ).
LT_SOURCE = LC_SOURCE->UPDATE( IM_NOMEI = 'X').
DESCRIBETABLELT_SOURCE LINESLV_SOURCE_N.
IFLV_SOURCE_N EQ1. "Assign the default data otherwise populate the list"
READTABLELT_SOURCE ASSIGNING<FS_SOURCE> INDEX1.
IFSY-SUBRC EQ0.
LS_SOURCE ?= <FS_SOURCE>.
LS_SOURCE_DATA = LS_SOURCE->GET_DATA( ).
MOVE-CORRESPONDINGLS_SOURCE_DATA TOLS_ITEM_DATA.
ENDIF.
ENDIF.
IM_ITEM->SET_DATA(
EXPORTING
IM_DATA = LS_ITEM_DATA ).