I think I already have this under another name: \<V, A\> void addAccumulator(String name, Accumulator<V, A> accumulator);
The method createAccumulator is private and only used for convenience methods. Please look at https://github.com/andrehacker/ozone/blob/c37e7ef6a83c2777c4753c1b783a283b7c5ce87d/stratosphere-core/src/main/java/eu/stratosphere/api/functions/RuntimeContext.java to get an overview of the visible API.
I also though about renaming this to registerAccumulator - do you want me to do this?