You would need to copy the line(s) and paste into a new MP file. Then use a converter. There is one for MP to GPX.
I contacted Adam at GPS visualizer
http://www.gpsvisualizer.com/He has so many cool converters there
I use a few of his converters and they are great
He is interested in adding this converter to his web site
Pretty straight forward if you just copy the source data in GPSMapEdit and then paste it into a converter window
Polyline-track data looks like this when it is converted to a .gpx track
[POLYLINE]
Type=0x10e05
Label=COURTNEY LN
Data0=(35.46245,-82.81889),(35.46284,-82.81923),(35.46297,-82.81953),(35.46322,-82.81988),(35.46322,-82.82022),(35.46314,-82.82031)
[END]
Converted Polyline
<trk>
<name>COURTNEY LN</name>
<trkseg>
<trkpt lat="35.46284" lon="-82.81889">
</trkpt>
<trkpt lat="35.46284" lon="-82.81923">
</trkpt>
...
...
...
...
</trkseg>
</trk>