[BaseCamp] How do I join routes?

[John Zabelle]

For the life of me, I cannot figure out how to join to previously created routes.  
For instance, I have a route from Molino, Florida going generally northwest connecting all of the Mississippi Tour of Honor locations.  I also have a route running roughly west to east across Tennessee connecting each of that state's Tour of Honor locations.  How do I make that 1 continuous route?
Surely this is do-able, right?  How??
I'm on a Mac, using the current version of Basecamp (4.4.6).

Thanks,
John Z
Molino, FL
Sent from my iPad