Make amphibious units?

I was wondering if there is a way to create units that can move on land and sea? Is there a way to create these units from the protoy.xml file? It occurs to me to add an option to transform units like in Age of Mythology (from thechtree.xml in this case), but I think that in the case of aoe3de the transformation is irreversible. Does anyone know of a way to achieve this?