I don't think you can do this using Bob. I'm not sure how to convert Attila units as I haven't modded this game.
This unit should be in the data\variantmodels\units. If you can't see this file create a file with the same name using Tweak.
It's possible that these factions don't have a unique daimyo model but I can add one by editing the relevant DB tables (campaign_character_art_sets, campaign_character_arts, agent_uniforms, frontend_faction_leaders).