Convert CSO to APK

Converting .cso to .apk (Compressed ISO Disk Image to Android Package File) appears to be impossible. You might try to repack and rename the file, but that still would not be of much use to you, since the resulting .apk file will be considered invalid in Android.

In the case you are attempting to convert one installation package to another, it might be more useful to contact the original developer, and find out whether they also provide a version of the specific installation package for Android.