In fact I converted two. One from Cossacks, one from American Conquest.
The question is not "modern technology", it's more to have animations that fit the Civilization III frames, or not.
Animation and scale for Cossacks and American Conquest, are not the same, so it may look a bit weird when played in CivIII.
However, these animations make good placeholders.
I myself will replace them if a unit maker would make better versions.