summary rows are supposed to be read only. Please specify the purpose of adding a drop down to it.
Anyway dropdowns can be added to a column in summary row by adding the following editor definition to the column.
editor: function(ui){
if( ui.rowData.pq_gsummary ){ //if summary row of grouped rows.
return {
type:'select',
options:['a','b','c']
}
}
}