You can make sprites for new units in RTW and BI by using a function that is built into both games by CA. To activate the function you need to add -sprite_script to your command line. For some general discussion on the process see here.
As well as the command line you need a text file in your RTW root level directory (same one the .exe files are in) with all the units that you want sprites for listed see example below
horse_light romano_british horse_light no_faction west_spearmen goths west_spearmen slave
you just need the units name as listed in descr_model_battle.txt followed by a space and then the faction name that you want sprite for (you must have texture line for that faction in DMB). Use no_faction for merc textures.