It's possible to upload gpx track files.
Unfortunatly the Import function is only available in the garmin connect Web application and I miss this funtion on my Android garmin connect mobile application.
1. To create gpx tracks I use the Android App 'simple gpx Editor'. Maybe someone know a more advanced Editor?
2. After creation I open garmin connect Web application URL with e.g. CHROME browser to import the gpx track file.
3. At last use the Android garmin connect mobile app to download, the now available, gpx track to the watch.
Try it, it works great.