RxJava - Any way to combine scan and flatMap?

Let's say I have a function that takes a String and a long and returns a Single<String>.

Single<String> stringAddition(String someString, long value) {
  return Single.just(someString + Long.toString(value));
}

Now I have this Observable...

Observable.interval(1, SECONDS)
  .scan("", (cumulativeString, item) -> {
    // Need to return the result of stringAddition(cummulativeString, item)
  });

I'm at a loss on how to do this. Scan needs me to return a String, but II would like to use the method that returns a Single<String>. To me it seems like I need something that can combine the behaviour of both scan and flatMap. Is there any RxJava2 wizardry that can help me?