My brother played Call to Power. I never had the, um, pleasure. (A websearch reveals that Call to Power's similarity to Sid Meier's games is coincidental, not intentional, though what little I saw of it reminded me very much of Civ II.) As for Alpha Centauri/Alien Crossfire...oh, yeah! Now, you guys are asking if it's possible to adapt those sprites or playing pieces to Civ III. Rightly so, it would not be possible. The orientation of the figures would be wrong, and the shading and coloration would be not nearly as clear and crisp. It'd be like comparing 16-bit color to 256-bit color. Similar, but different.
But no matter.
I should think that every unit in CtP has an equivalent somewhere in the Unit Graphics forum. I've seen battlemechs and hovertanks, spies and civilian specialists, machines and vehicles for landscapes and moonscapes, and who knows what else? Why would anyone want to go through the effort of adapting those 1990s-era gifs (or whatever they used) when simply downloading a ready-made unit would work so much better? Provide me with a list of units (and a pic or brief description) and I'd give you a list of CivFanatic equivalents. I tried to do just that instead of this reply post, but unfortunately no unit list or photo thereof was easily located on the World Wide Web.