1. Click your 'Android Apps' folder
2. Click 'Open In Finder'
3. Right click an app (e.g. Air Control Lite) and select 'Duplicate'
4. Rename the new app
5. Right click the app and select 'Show Package Contents'
6. Open the 'Contents' folder
7. Drag 'info.plist' to a text or plist editor
8. Find the '<key>GuestAppActivity</key>' and '<key>GuestAppPackage</key>' entries.
9. Replace the package names with your desired app (e.g. com.zinio.mobile.android.reader)
10. Double click to test!
Unfortunately BlueStacks is b0rked on my Mountain Lion install so these instructions are from memory, so let me know!
Installing a file explorer or the Dropbox app would probably be a good way to sideload apps.