Maybe someone else will be able to help - I'm not sure...
but if the '.fit' format is well documented then it would probably be pretty easy to add it as another format that the 'recorder' app can export to
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Maybe someone else will be able to help - I'm not sure...
but if the '.fit' format is well documented then it would probably be pretty easy to add it as another format that the 'recorder' app can export to