[OSM-newbies] Can't figure out how to upload a park boundary to OSM

[Hilton Long]

I've extracted a boundary definition for Capitol Reef National Park from
Tiger line files, 2006.  It's in gpx format, and shows as a continous,
closed line with numerous vertices.  I'm new to OSM, but I think this should
show up as an area feature if I could figure out a way to import it.
Unfortunately, I can't. 

This is really frustrating, because I see an area indicating the boundaries
of Zion National Park just east of Saint George, UT, USA. 

OSM rejects my gpx upload because it doesn't have time stamps.
JOSM opens the the gpx file, but only shows points.
Is there a way to get JOSM to open shape files, or to convert the gpx files
to ways, the way the slippy map does when it likes the gpx I upload?

Thanks, 

Hilton