// Will place the new column after the `another_field` column:

$fields = array(

     'preferences' => array(

       'type' => 'TEXT', 

       'after' => 'another_field'

     )

);

​

// Will place the new column at the start of the table definition:

$fields = array(

    'preferences' => array(

      'type' => 'TEXT', 

      'first' => TRUE

    )

);