[OSM-newbies] Adding a cul-de-sac to an existing street

[Isaac Wingfield]

Park Royal Drive, here: http://osm.org/go/TZMgvYcdU-

has an unmapped cul-de-sac sticking out to the side, which you can see  
in Edit mode. There is no separate street sign there; it's a part of  
Park Royal Drive.

Using Potlatch, I cannot figure out how to add that such that it is  
not a separate entity (it should be a part of Park Royal Drive).

How do I do that?

Isaac