Well ok,
After you create/launch cl_salv_hierseq_table=>factory , put this code
TRY.
lr_columns = lr_hierseq->get_columns( 1 ).
CATCH cx_salv_not_found.
ENDTRY.
*... set expand column
TRY.
lr_columns->set_expand_column( 'EXPAND' ).
"This EXPAND it's your checkbox "field name
CATCH cx_salv_data_error cx_salv_not_found.
ENDTRY.
*... set items expanded
TRY.
lr_level = lr_hierseq->get_level( 1 ).
"lr_hierseq is the IMPORTING parameter of your
"cl_salv_hierseq_table=>factory method
CATCH cx_salv_not_found .
ENDTRY.
lr_level->set_items_expanded( ).
" lr_level TYPE REF TO cl_salv_hierseq_level
Hope to help.