The Japanese write Muhammed like so: ニ筑ニ築ニ陳?歎ニ暖 In romaji it's: Mohameshito. I got this info from the enamdic file. The closest you're going to get with real Japanese writing (without kanji) is probably: ニ停ぎニ地ニ陳?檀 In romaji: Muhamedo. Of course, the Japanese don't write it like that. So my guess is to go with Mohameshito, even though it doesn't sound at all like Muhammed. The next step is probably to use nanori (kanji readings for names) to swap for the kana. I must admit that I don't know Japanese (I just have some knowledge about the writing system itself), but this might start you off. By the way, it doesn't seem like a good idea to just use a kanji with the same meaning as your name, you really should use nanori if I'm correct.