Rich Notes supports the iOS Shortcut feature. This allows you to define text abbreviations to be expanded automatically to full text. To enable:
Launch the Settings app on your device:
Settings > General > Keyboard: Shortcuts
Rich Notes does not support the TextExpander app. While it's popular, I've decided to stick with iOS shortcuts which are free and available to all the apps on your device.
If you feel strongly about the need for TextExpander, drop me a note.