I'm using KeyNounceBengali app. It's really a good app but don't know how to type Bengali full stop which is similar look like "I".
I didn't see any special instructions to achieve this since the software is actually using the transliteration method to type in Bengali with the help of an English keyboard. I've checked the application info and it doesn't appear to be a different method. Simply switch between alpha numeric keys and symbols and you will see the desired key that has to be pressed in order to have the full stop.
If you have issues, reinstall the application through the following link: https://itunes.apple.com/us/app/bengali-transliteration-keyboard/id971949939?mt=8
