The Twiddler does not store any letters or numbers. It only stores HID Codes. These codes are sent to your host when you press a chord. Your host determines what to show when it receives the HID Codes.
What you are describing is entering a Unicode code. This is much different than HID Code. The Twiddler can handle Unicode input. You just have to emulate what you do on a regular keyboard.
In Tuner, instead of typing the Unicode (Alt-0233) in Tuner to get "é" to show up in the Tuner text field.
Type the keystrokes you would use for the Unicode. For "é" type Alt-0233:
<LAlt><KP0><KP2><KP3><KP3></LAlt>
Now every time you press the chord it will send out: Left alt–keypad 0-keypad2-keypad 3-keypad 3- close left alt
Your OS will interpret that as a Unicode command and print "é" on screen.