is it possible to override speed with own calculation ?

I have  hooked up a cadence sensor to my exercycle which is great because  now I get RPM on  my garmin watch. Of course I still dont have speed or distance.However I was wondering is it  possible to override  activity.info   current speed with a calculated speed  based off my cadence ?  or  this not allowed and  would i need to write a custom FIT data field instead?