Hi John - yes, you can join several routes together on plotaroute. Select Combine Routes from the menus. Please note though, this feature is only available on the desktop site not on the mobile site.
A friend of mine recently showed me how at GPSVisualizer.com he could combine multiple tracks into a single GPX file, so before I start trotting over there for this single useful function, I wonder whether Plotaroute can do it too.
Rather than having to load say 10 separate tracks for a park I'm going to, loading a single combined file would be a lot tidier way to manage it.