There is a Sysinternals tool to create a VHD (which is the standard format for a drive used by VirtualPC/Hyper-V).
Disk2VHD
##Disk2VHD
http://technet.microsoft.com/en-us/sysinternals/ee656415.aspx
I assume you could add to it at a later date although I haven't confirmed this.
For some reason 'volume shadow copy failed' for me but I was trying to backup an external drive that wasn't my boot drive so I could safely just turn it off.