You can pair your Garmin device to your sensors and still use The Sufferfest App. This will allow you to record data on your Garmin and still view it in the App. The Garmin device will not interfere with The Sufferfest App, but it will also not add anything to The Sufferfest App.
You can upload the workout data from The Sufferfest App to Garmin Connect. Sufferlandrians have the option to have their workout data emailed to them via a .tcx file.
After you receive all of your Suffering data via email, it is as simple as going to Garmin connect and uploading your data.