This thread looks to be a little on the old side and therefore may no longer be relevant. Please see if there is a newer thread on the subject and ensure you're using the most recent build of any software if your question regards a particular product. Otherwise - post away!
You might be able to just copy and paste the XML in a text editor. You'll need to reconstruct the diagram though. Also, if your models have the Name attribute set, you'll need to set them all to the same Name before doing the copy and paste (do this in the designer, NOT in the XML, because it's the side-effect of updating the internal entity monikers that's important).