You shouldn't have to call StopWatching, you should just be able to call SetWatchedSymbols() while it's getting data.
In practice I don't think we do this in RightEdge, and looking at the source code to the IQFeed plugin it looks like this will cause the symbols you are already watching to be requested again. I don't know if that's a problem or not. The source code to the IQFeed plugin is available
, so in any case you can experiment yourself.