[OSM-newbies] What's the easiest method to "snap" an area boundary to adjacent ways?

[m902]

  I have a few situations where the boundary (or part of it) of an area 
is an adjacent way. Examples are:
- A sports field, bounded by a river on one side and a road on another.
- A residential area bounded by a road on one side and the sports field 
on another.
- A commercial area bounded by a road.
Obviously I could draw the boundary by selecting each node of the 
adjacent way...but I wondered if I am missing somethng and if there is 
an easier method?
I would like to just select a beginning node and an end node on the 
existing way and tell it to set the area boundary to follow it.
Is anything like that possible?
Couldn't see anything like that in Potlatch or JOSM.
Thanks...
Martin