I don't know that a list exists. but I would think that you would want someone with at least basic skills in sql, microsoft server and IIS as well as web design if one person is doing the whole thing. You would also want someone that has implemented a DNN site in the past. By extending it you could mean adding modules which is easy to do, or developing modules which requires programming knowledge as well as more advanced skill in the first areas I mentioned. If they are well rounded webmasters, they should be able to learn along the way as DNN is installed and run successfully by people that had no previous experience with it (myself included).
Mike

 If they're asking you to extend it, it definitely seems like you're going to have a significant number of questions about how to create or implement custom modules.  There are, believe it or not, some decent papers out there on creating dnn modules.  Of course most of them are different and none of them seem to be 100% correct... but this one is ok:

http://www.15seconds.com/issue/050505.htm

Perhaps also expect questions about user and role management and which providers dnn "supports."

Since dnn is written in crappy visual basic, you might also get some questions regarding the use of other languages and how you can include c# code files in your project. (you have to modify the web.config file)