A little history for you and others:
AppToMarket was developed because, initially, MIT AI, didn’t allow publishing apps to Google Play Store. Users used AppToMarket to make their apps publishable by creating keystore certificates.
Later MIT AI made their apps publishable. Then AppToMarket was updated to allow updating the AppToMarket keystores so that it would be compatible with App Inventor. You can see that through here:
Now, checkout link below that discuss how to make your keystore compatible with AppyBuilder.
NOTE: Use export to backup your existing AppyBuilder keystore