Is your NSDictionary full of other NSDictionaries? It sounds like you would need an NSMutableArray that you could add NSDictionaries to at the end. Assuming you can control the flow of data coming in and wouldn't run the risk of duplicates in your array, you could certainly append the data.
Without seeing how you are implementing it, I can't provide more detailed code examples. Appending items is easy to do, but know it can only be done with mutable versions of Arrays or Dictionaries. Hope this helps a little.
Use NSMutableDictionary addEntriesFromDictionary to add the two dictionaries to a new mutable dictionary. You can then create an NSDictionary from the mutable one, but it's not usually necessary to have a dictionary non-mutable.