Since you can only use the 'Send' activity in a microflow, you will need to call a microflow from the nanoflow in your native app. You can access the data that was synced from the device to the server by retrieving it, or use a non-persistent entity to pass the data from the mobile device directly to the microflow. Copy the data from the non-persistent entity to a new or retrieved external entity and send it.
Another scenario could be to use an after-commit event on a persistant "helper" entity. The helper entity can have the same attributes as the external object. Just commit and sync, and copy the values to the external entity in the event handler flow. This method will give you less control so proper error handling is recommended.
P.S. I do believe Mendix is working on making this a lot easier in the future but have not communicated any official timelines.