Hi Martin,
Not quite sure what you are looking for, below listed the example code from the BAdi own example class.
FIELD-SYMBOLS: <l_s_data> TYPE data. LOOP AT c_t_data ASSIGNING <l_s_data>. if_ex_rsroa_olap_badi~compute_single( EXPORTING i_s_rkb1d = i_s_rkb1d i_partcube = i_partcube i_ncum = i_ncum CHANGING c_s_data = <l_s_data> ). ENDLOOP.
As far as I know, you can access c_t_data with field symblo like the code listed above.
Regards
Bill