Is it possible to add other networked sources to the Apple Dictionary program?
There are a few things you could do, but none of them are exactly what you're asking for. If you have a dictionary in the StarDict format, you could convert it for use in Dictionary.app with DictUnifier. You could also develop your own dictionaries, perhaps using the project template available as a supplemental download to Xcode.

Those would create static dictionaries, though, not a "live" dictionary source like we have for Wikipedia. Wikipedia has special support files built into the Dictionary.app package. Cheaters!

My approach to this kind of thing has been to use the web search module in Quicksilver to add every dictionary I might want to check against (Oxford English Dictionary, Wolfram|Alpha, Black's Law Dict., Google "define word, etc.), along with a keyboard shortcut that sends QS any selected text (or I can just bring up QS and type my search directly). Alfred, et. al., probably also have a similar feature.

This approach only searches one dictionary at a time, but usually I know which one(s) will give relevant results. Results come up in your browser; all the QS web search module does is format the URL for each source to perform the appropriate search. Also, since we're not using Dictionary.app these dictionary options will not appear when you CTRL+CMD+D on a word. (In fairness, neither does Wikipedia!) All that said, this approach requires minimal setup and is really quick.