Welcome to Android Central! One way to do this:
1. Go to Settings>Language & Input, tap Google Keyboard, then Dictionary>Personal Dictionary>English.
2. Tap the + sign at the upper right to add a new entry.
3. Type the word you want to enter into your personal dictionary.
4. Add the shortcut (like "yh" for your yahoo address).
Now, when you type that shortcut, you'll see the full word appear in the suggestions.
Edit: Javier P, a fellow Ambassador, just informed me that you can't use special characters in the shortcut, so your example of "m@" wouldn't work. Thanks, Javier!