Yes it is. You must edit battle_models.modeldb in Medieval II Total War\mods\Third_Age_3\data\unit_models !
Just replace model for unit/s you want to add new look or export_descr_unit.txt to add existing one !
bold red numbers must be exact number of characters in line, (use word or similar program to count)
easy way is to replace just wich model unit is using in export_descr_unit.txt (data) , under category armour_ug_models just put the one you want (if they exist)
Hope this help, there are tutorials explaining the meaning of code inside battle_models.modeldb look it up !
To add look of Dragon Slayers just put under category armour_ug khazad_dum, khazad_dum_upg
*sorry for bad English