Also, you can ask the creators of Warlord for permission to use their own works.
I agree. Warband is fun to modify, but its time is over if one wants to create a project for it. The playerbase has greatly diminished recently, so not many people would get to see one's shiny new modification. It is better, in my opinion, to stick to existing projects which still attract players and enjoy moderate success despite the shrinking playerbase of Warband.
Not many would go back to Warband with Bannerlord assets, because it would prove to be a step backwards into boredom after a decade of playing only the former game and its reincarnations/ different modules with the same mechanics core. But porting Bannerlord items to Warband is clearly executable in a way MadVader described. The appeal for such a modification might turn out to be scarce, as the nucleus of Warband playerbase that stay with said game cannot move on due to their gear which cannot handle Bannerlord. That being said, they might see no reason to buy Bannerlord only to have a game they cannot run for the sake of playing a mod with Bannerlord assets.
A Moderator and a Community Manager pointed out to a principle of the company which had made both games. It stays true, because one cannot use Bannerlord assets in Warband. Reiterating my previous post, you may create the items from scratch and use Bannerlord assets as a point of reference, which are available to behold, but not to port to Warband.