Funny that you should say that as there are dozens of kanji for 'jo" , for 'na' or for 'su" . You could write it like that : 助茄子 but it means "help aubergine", which doesn't really make sense. What about that ? 序奈巣 "order +'na' phonetique + simple". l'avantage de ce dernier est que le 巣 resembles very much 単 (tan) and therefor could be mistaken for Jonat(h)an, which I believe is the English, German or French version of your name.
I believe "Jonas" is from the Hebrew "Jonah," and John or Jonathan the English equivilant. By my count anyway, there are at least 11 most appropriate ways to express Jonathan （Jonansan）in kanji (譲納餐 probably being the best because it is a phonetic rendering that still retains much of the original meaning).
But it all depends on your preference for the purely phonetic, denotative or a combination of the two.
For "Jonansan" (ジョナンサン), or a close variation, under a phonetic/eulogistic transcription you could alternatively be known as：
"one who gets rid of bad fortune" "(除難散）
"a brilliant lover" （情男燦）
"one who helps names be known to the world."（助名残）
or "one who gets girls by praising them" (女納讃）
I'll try to type some more in when I'm in a position to do so.
For John (助運、慈預運、示誉恩), "Jiun", Jiyoun" or "Jiyoon"（ジョン）You could fashion yourself：
"a samari's grudge that continues for generations" (侍代怨）
"one who brings luck" (侍与運）
or "benevolence which brings luck" (慈与運）。
Or there are some less kind ones if you'd like to choose from those as well.