RXJava emmit from asynchronous source

[Jadefalke]

Hello, not sure if this is the right place to ask, but I was wondering how you could asynchronously add items to an Observable for it to emmit. More specifically I have a method walkFiles which should return an Observable<Path> but I'm not sure how I'd go about returning the observable while still adding items to it asynchronously in a different thread. 

Maybe I'm missing the point of RXJava here but I figured I'd ask. Thanks for the help