This was something that I really missed when I upgraded to 10. To install go to Customize -> Customize Mode -> Add from file... and select the ModifyWithSketch.tlb file in the folder ModifyWithSketch\bin\Debug. Then you will have a new tool in the Commands tab under "Developer Samples" called "Continue sketching an existing..". You can drag this to any toolbar. To use, just select an existing feature and then click on the tool.