Примерно так:
$attribute_set_name = 'your attribute set name here';
$group_name = 'your attribute group here';
$attribute_code = 'your attribute code here';
$setup = new Mage_Eav_Model_Entity_Setup('core_setup');
//-------------- add attribute to set and group
$attribute_set_id=$setup->getAttributeSetId('catalog_product', $attribute_set_name);
$attribute_group_id=$setup->getAttributeGroupId('catalog_product', $attribute_set_id, $group_name);
$attribute_id=$setup->getAttributeId('catalog_product', $attribute_code);
$setup->addAttributeToSet($entityTypeId='catalog_product',$attribute_set_id, $attribute_group_id, $attribute_id);