When you create a body with Extrude command, click with Right Mouse Button on an end arrow of the extrude. In the list, you will have Snap to Object. Select a point somewhere on a body, and extrude will be created up to this point. But beware, this is not an associative connection. If the selected points move, the extrude (that used this point in Snap to Object) will not move.
Attahced is the movie of how this is working (If this is what you have been looking for).