Ha, the potential conversion of Civ III and Civ IV units has been discussed before, but it never got very far, because they're so fundamentally different. Civ III units do use textures and materials, but not skins in the same way as Civ IV units. Civ III units are fundamentally 2D graphics, although you usually create them with a 3D graphics program. That's why they can be as detailed as you like, because they are pre-rendered.
[Chugginator] I forgot you posted on my thread before - can't think how I didn't remember an avatar like that! I was thinking only yesterday that it would be fun to make an Ichigo unit.