It MIGHT be possible to simulate a bluetooth keyboard (see also HID - human interface device), so the Arduino reads the sensor values and the "types" the data on a keyboard.
As Pylon said, I doubt you will be able to get into the source code for the commercial app. Does it have the ability to interface to real sensors? If so you might be able to use the Arduino to simulate the regular inputs.
You can write your own app, which takes the data from the Arduino and performs the same calculations or use a lookup table to get the desired results. I am just starting down the path to writing Android apps, so I can't give any good advice, but here are a couple of tutorials I am looking to for guidance.