A bit of pseudo modelling here, but assume the following domain model based on the provided json structure:
In a microflow:
Action 1 is the retrieve of the list of innerobjects
Action 2 creates an object of type lineitems and the list of innerobjects is associated like below:
Action 3 creates an object of type parentObject and the newLineItems object is associated like below:
I think you need to change the domain model to allow for this, but I hope this points you in the right direction.