SAP/ABAP: Add new buttons to selection screen beside standart ones

AT SELECTION-SCREEN OUTPUT.

  SET PF-STATUS 'PFS'."Create whatever you want here...

AT SELECTION-SCREEN.

  CASE sy-ucomm."control whatever you want...

Another way;

INCLUDE : <icon>.
TABLES: sscrfields.
DATA: l_sel_button TYPE smp_dyntxt.

INITIALIZATION.
  l_sel_button-icon_id = icon_start_viewer.
  l_sel_button-text    = 'Edit recipient list'.
  l_sel_button-icon_text = 'Edit recipient list'.
  sscrfields-functxt_01 = l_sel_button.

AT SELECTION-SCREEN.
  CASE sy-ucomm.
    WHEN 'FC01'.
*      PERFORM start.
  ENDCASE.