To add values to inner collection you need to create another collection similar to inner collection with all fields same as inner collection and then all the values to the new collection and at the end just add the new collection to the Main colllection which contains inner collection
EX: lets say ""MAIN"" is name of collecton which contains one of the field of type Collection named as ""INNER"".now create a new collection as NEW and add all values to it and at the end with the help of a calculating stage just Store ""NEW"" in ""MAIN.INNER
