Not unless you have access to the original models. Civ3 units are assembled and animated in 3D software like Blender, but then converted to 2D sprite sequences. Ironically there's a lot of Civ4→Civ3 conversions (including some leaderheads), but otherwise none of the leading Civ3 artists want anything to do with 4.
From the sound of it, your best bet would probably be Freeciv. It takes a fair bit of exploration to figure out how everything works, but once you know how the individual rulesets communicate with each other adding new units and improvements is about the same as Civ4. The biggest problem (besides criminal lack of n00b-friendly documentation) is the engine is constantly being updated in ways that can totally rewrite coding definitions, and as of yet there's no automatic upgrade tool for outdated mods.
From the sound of it, your best bet would probably be Freeciv. It takes a fair bit of exploration to figure out how everything works, but once you know how the individual rulesets communicate with each other adding new units and improvements is about the same as Civ4. The biggest problem (besides criminal lack of n00b-friendly documentation) is the engine is constantly being updated in ways that can totally rewrite coding definitions, and as of yet there's no automatic upgrade tool for outdated mods.