select multiple header rows

You are only getting one row because you are only referencing the first row, you need to loop through the rows (using the header row count as a counter). See below:

myTable = app.selection[0].tables[0];
myHeaderCount = myTable.headerRowCount;
for (i=0;i<=myHeaderCount -1;i++)
{
        if (i==0) 
        { 
                    myTable.rows.select();
        }
        else
        {
                    myTable.rows.select(SelectionOptions.ADD_TO);
        }
}