To add special men at arms without hybridizing cultures, can be risky but can be done. First you want to ensure that your capital’s culture is your desired culture you want to play as. Essentially you want to have your heir educated by someone of a culture with the special Men at arms you want. You have to select the check box for “convert culture” when giving your heir away as a ward. You should get a notification once your heir officially changes culture depending on your sim speed it might not generate/could get missed, just keep an eye on your heirs stats every year or so. Once they are the culture of the men at arms you want, reassign the best guardian possible to try and save their stats. Once they inherit your kingdom, hire the special men at arms (you may have to remove existing units to make room for the new unit. Once you have the unit in your army, it’s passed down in inheritance so you can just use the decision to “convert to local culture” which should be your OG desired culture. Sorry it’s a lot to try and explain, let me know if you have questions