[TYPO3-mvc] where do I set "CONFIGURATION_CREATION_ALLOWED" to TRUE?

[Tim Schoch | GSTALTIG]

I want to create nested objects in the same edit form and get the above mentioned error.
Structure:
parentObject
- 1:m childObjects (storageObject)
  - 1:1 childChildObjects

Input names look like this:
tx_myext[parent][attribute]
tx_myext[parent][childObject][0][childChildObject][attribute]
tx_myext[parent][childObject][1][childChildObject][attribute]

The mapping paths seem to be correct, atleast the error message has found the correct attributs 
where the new objects should be created:
« Exception while property mapping at property path   "datas.0.dialSeat":Creation of objects not allowed. 
To enable this, you   need to set the PropertyMappingConfiguration Value   "CONFIGURATION_CREATION_ALLOWED" to TRUE»
( datas = childObject, dialSeat = childChildObject )

Is is not possible to go two levels deep?

Thanks, Tim