adding garmin extensions

[Sprechen]

I need to add information to my trk infos, specifically I need to add the speed of each point in the segment. This is defined by the garmin extension https://www8.garmin.com/xmlschemas/TrackPointExtensionv2.xsd. As I understand, for each point I can use the extensions map to add additional data for each Wpt point, but how I can add schemas to the Gpx node?