You need an extra application for this - Virtual Audio Cable.
Virtual Audio Cable creates a virtual sound card. You can then create the following path:
Mic -> Stereo Tool -> Virtual Audio Cable 1 -> Skype
Note that there are some issues with Virtual Audio Cable if you're using Vista...
The Virtual Audio Cable site:
Last time I checked the "free" version yells "trial" every few seconds in the audio it outputs, so you'll probably have to register it (around 21 euro).
Some (not many) sound cards offer similar functionality in their drivers, so that might be worth checking out first.