if you have a nested repeater you can nest that in the array
$value = array(
// element for row
array(
// field in parent repeater
'field_12345' => 'value'
// nested repeater
'field_23456' => array(
// element for row in nested repeater
array(
// field in nested repeater
'field_34567' => 'value for field in nested repeater'
)
)
)
);