I've developed an Android alarm clock/reminder app and I'd like to know if there is a way possible to make Google Now at least prompt the user whether they want to set the alarm/reminder either through the default app or mine.
What you describe sounds achievable with Google System Voice Actions. It is a bit different than Google Now, the anticipatory cards UI, but as Google Now is shown with the speech recognition part that may explain the confusion when searching.
See: Google System Voice Actions
Unfortunately only a fixed set of voice actions (system actions) currently are supported: https://developers.google.com/voice-actions/custom-actions