yes.
There are several ways to accomplish this, with different degrees of difficulty and different issues created by it.
1) different units types. you can write a script that when a certain event/action occurs it changes the unit type of the effected unit, thus allowing you to have different graphics and unit abilities.
2) an ability was added in BtS that allows you to change what Unit art is displayed while keeping the unit type the same. this is used for ethnically diverse units in the game and in mods.
3) the third and most difficult method is used in the Afterworld scenario. when creating unit graphics you can create them in such a way as the graphics will toggle parts on and off with the addition/removal of promotions. this one is not able to change the animation used.
the difficulty of the first 2 methods is that you need to generate a separate nif(unit art) file for every version of the unit. so if you want one specific hero to display an axe, that requires 1 additional unit, but if you want ANY unit to display the axe it will require hundreds of new units.