I don't run this, sorry. But you can get idea.
DB::select('a.row1',array(DB::expr('a.row2+a.row3'), 'TOTAL_SUM'), array(DB::expr('MAX(a.row4)'), 'MAX_VALUE'))->from(array('some_table', 'a'))->execute();
And read document: http://docs.fuelphp.com/classes/database/usage.html