Well if you are going to port an app that was made from a specific platform , i would suggest to make them according to the platforms framework and do not copy much about the UI and flow of it cause it is different on any mobile platform. And you can only use the business flow and system design of it!
There are no tools to convert, nor should you even trust them. There is too many differences between platforms. The only write once run anywhere mobile app is a web app version, but there are a lot of trade offs to this approach.
For a web app look at PhoneGap, it is a good library for creating a non-native app that runs on many types of mobile platforms.
Me personally, I feel I would re-write the app on each platform to get the UI experience to be the best it could be. I don't like choppy web apps on a device, not a great experience in my mind.
Since Steve Jobs has relented on "The Frameworks" question (i.e. You're allowed to use them again) you can write for those frameworks and go from there.
If you're doing 2D (non OpenGLES) development, look at Corona. It will target iOS and Droid. http://www.anscamobile.com/corona/ There is Unity http://unity3d.com/unity/publishing/ios.html And I am positive there are at least 10 more you can look at.
I agree with rewriting the app for each platform. Once you are comfortable with that, then try one of these engines. That way you'll understand any problems that crop up due to the framework that you have selected.
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