Now that lemmy101 has released his (and CaptainBinky's) Nexus patch and material hacker (see
here) it is theoretically possible to do this. It's not going to be very easy or fun though.
Roughly speaking the conversion process would be something like this:
1. Import Civ4 NIF and animation KF files one by one into Blender and export them as FBX files. This will have to be done individual for each animation so if the unit has 25 you have to do it 25 times.
2. Use Nexus Asset Import to convert the all FBX files to GR2 files. There should be one gr2 for the main mesh and then several animation files.
3. Use the material hacker to add the textures to the mesh GR2.
4. Make an fxsxml file that lists everything mesh, animation, textures, bones for the converted unit.
5. Make a mod includes all the files and points to your fxsxml.
6. Try in Nexus and then in game and see if it works.
Even then you are faced with many complications:
1. NIF -> FBX does not seem to be perfect in blender at least. I tried with and archer and the bow was completed mangled by the time I got to the GR2.
2. Another big problem is that the animation sets between Civ4 and Civ5 are not one-to-one. Civ5 has a lot more. Some of the animations in Civ5 that do not exist in Civ4 are:
IdleA_Trans_IdleB
Shuffle
Turn_Left
Turn_Right
Stop_Run
Stop_Run
Charge_Run
Charge_Attack
AttackCity
Victory
Bombard_Defense
Bombard_Defense_Idle
Bombard_Defense_Trans_IdleA
These would have to be created by hand (which is pretty unfeasible) or substituted with a Civ4 one (which will probably look weird).
3. Attaching effects and projectiles could be difficult - you'd have to use the vanilla Civ5 ones I guess which could be difficult.
In short, it is not going to be straight-forward, and I'm beginning to think it is pushing the boundaries of feasibility.
It would be much better if we can rig Civ4 meshes to Civ5 skeletons and animations. It may be possible to get the Civ5 skeletons out with the work lemmy101 and CaptainBinky have done. It is probably worth exploring this solution first, since this is how the majority of Civ4 units were made.