//まずは初期設定オプション。
$default = array(
    'conditions' => array(
        'teller_id' => 0,//無料占いのみ
        'subcontact_count <' => 3
    ),
    'fields' => array('Contact.id','Contact.user_id'),
    'order' => array('Contact.id' => 'desc'),
    'limit' => 20
);

Array
(
    [conditions] => Array
        (
            [teller_id] => 0
            [subcontact_count <] => 3
        )

    [fields] => Array
        (
            [0] => Contact.id
            [1] => Contact.user_id
        )

    [order] => Array
        (
            [Contact.id] => desc
        )

    [limit] => 20
)

//後で追加したい配列
$newoption = array(
    'conditions' => array(
        'teller_id' => 5,//無料占いのみ
        'subcontact_count <' => 17,
        'new' => 'new'
    ),
    'fields' => array('Contact.user','Contact.name'),
    'limit' => 17
);

//これを合体
$q = Set::merge($default,$newoption);

//すると後に指定した配列のデータを優先して配列を結合させる
Array
(
    [conditions] => Array
        (
            [teller_id] => 5
            [subcontact_count <] => 17
            [new] => new
        )

    [fields] => Array
        (
            [0] => Contact.id
            [1] => Contact.user_id
            [2] => Contact.user
            [3] => Contact.name
        )

    [order] => Array
        (
            [Contact.id] => desc
        )

    [limit] => 17
)