You can accomplish this by creating an extra java action that takes a string as the input. I've done this previously, and called it "postStringDataWithResult":
In your java action code, you just need this one line of code:
return RestConsumer.postObject(getContext(), collectionUrl, null, dataString, responseData).getMendixObject();
EDIT: This will throw compile errors, as my modified code also had adjustments to the "postObject" and "request" methods in RestConsumer.java
Here is a link to the modified files you should need, including the new java action: here.
Once you've got this set up, in your microflow, either:
Finally, pass that into your new java action.