Either I do not understand your reply, which is likely, or you misunderstand my point. Regardless of whether the English noble is given temporarily or permanently, the fact is, it is from another faction. So unless I am further confused, it means he came from another file, which means that the co-mingling - mixing- of faction units is possible...No?
In game files term, he's not from another faction, he's part of the Teutonic Order, but just looks English, has English name, and has English looking units. That's it. As far as the game's concerned he's part of the Order.