[lisa]

Load the original model that you wish to use as reference. Select everything and click "lock". Sometimes its easier if you make the locked object solid, which you can do by going into the main settings and unchecking "Show locked objects as wireframe".

Then start building your new model on top of the old one, using the old one as reference. The vertices won't snap to the surface like they would in a retopo tool--you'll need to align them visually--but otherwise the process is similar. Start clicking out polys and extruding edges until you've basically made a sheet on top of the old model. It's easiest if you work in sections, then use "create ordered surface" to bridge the sections.