For iTunes on Windows, you can use this guide to add content to your iTunes library: https://support.apple.com/kb/PH20507?locale=en_US
For iTunes on Mac computers, you can use this guide to add content to your iTunes library: https://support.apple.com/kb/PH19613?locale=en_US
For iPad or iPhone, Apple does not allow import of audio files that are not sold in the Itunes store. We continue to look for alternatives and will post them, if they are found. You may always stream the audio content when you are connected to the internet.
If you need further assistance, please submit a ticket to the Open Gate Sangha Tech Support Team: http://www.opengatesangha.org/techsupport