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Theoretically, the Application Programming Interface ( API ) of Jelly Bean is higher than that of ICS. Hence some features of the Jelly Bean apk may not work on ICS, or the apk may not even work. It depends on what you are trying to port. Some Xperia Z apps have already been ported for ICS, but other phone brand apks are very difficult to port.
You cannot mess with the API level. Although it can be changed in androidmanifest.xml, many features of the app won't work. Still, you can try using APK2JAVA or APKTOOL to decompile and check the apk that you need to port. If you don't know java, then it would be difficult.
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