If there is a need to create iterations or releases via the API it can be beneficial to use the built in cascading feature in Rally to propagate those timeboxes down through the project hierarchy.
Release : 2.0
Component : API FOR AGILE CENTRAL
In order to cascade timeboxes, add cascade=true to the "create" endpoint as shown below:
https://rally1.rallydev.com/slm/webservice/v2.0/iteration/create?cascade=true
You would continue to craft your JSON payload in the normal manner.