There are ways. When you do the restore, tell it not to erase the destination. I'm not sitting in front of Lion at the moment, so I don't know if they've removed that option. If they have, Carbon Copy Cloner also makes it easy. If permissions matter (e.g. it's a boot drive), make sure to enable permissions on both the source and destination before doing either of these.
If you're going to be doing something like this regularly and from disk images, there are proper ways to make the disk image.