The error you are getting is a common one when working with templates in Dreamweaver.
There is no solid fix for the issue, but I have discovered a workaround. See the following thread for details:
as far as adding other behaviors to an existing spry element, you will need to edit the spry behavior that is already applied.
if you select a spry element, you should notice a blue bounding box around it, click this bounding box, then go to the property inspector to edit the spry behavior.