Kevin Remde discusses the topic of Hot-Add in Hyper-V in today's server virtualization blog series article.  This is a great reference for those looking to get certified as well, since the article quickly identifies what can be added to a running VM in Hyper-V, as well as what cannot be added and why.  Give it a read here:

http://blogs.technet.com/b/kevinremde/archive/2013/03/05/you-want-to-hot-add-what-20-days-of-server-virtualization-part-2-of-20.aspx