I use this site for many types of conversions
http://www.gpsvisualizer.com/Also adding elevation to .gpx tracks that don't have it
The site has many cool conversions
Everything ends up getting added to my custom maps in GPSmapedit
Then converted to a Gamin .img file using cGPSmapper
As far as OSM, I edit my own OSM data using JOSM
You can save that as a osm file
If you register with OSM, you are then allowed to edit OSM maps
There are 3 ways to edit the OSM map data
JOSM allows you to add .gpx track data to the main map layer and then upload it
https://josm.openstreetmap.de/The screen shot below shows the edit menu