Whilst it is possible to convert APK to BAR so Android apps can be used on Blackberry Playbook, it is not possible to convert BAR to APK. So in short, there seems to be no way how to transfer BlackBerry apps saved as .bar files to .apk files for Android platform. That is because Android OS lacks the BB unique runtime.
You can find utilities like the BAR to APK Converter, but users report that it is not reliable enough and may not work for most .bar files.
Most developers of BB apps also provide a version for Android, so you can check the app's developer website for more clues.