if I have a native android app (written in Java of course), what cross platform tool would you recommend to make it usable on iOS as well? I have been reading quite a lot about it, but most toold require code being written in html,css,js (phonegap, appcelerator etc), or for instance c# (xamarin), but which one would wrap my java code?
There is no solution that will work out of the box. You'll likely have to rewrite at least the UI whatever way you choose.
If you want the code to run on any mobile platform, you'll have to rewrite your code in html/css/js.
If you just want it to run on iOS, you have these options: