Add a subquery to SELECT'd columns in a dynamic query
You can use db_query()

$result = db_query(
  'SELECT a, b, (
    SELECT x, y
    FROM {table}
    WHERE table.x = :value1) as c
  FROM {table_2} AS table_2
  WHERE table_2.a = :value2',
  array(
    ':value1' =&gt; 'value',
    ':value2' =&gt; 'value_2',
  )
);