This is called debugging. You obviously debugged a V3/V4, and no, unfortunately you can't use passwords, get a user name, etc. through a debugged Tamagotchi. However, a debugged Tamagotchi has some special features that more than make up for not being able to use this.... in my opinion. A debugged Tama, can:
1. You can choose any character, including secret-never-before-seen characters!
2. Can run in hyperspeed! Press down the C button to make your Tama move really fast. It stops when your Tama reaches a critical moment, such as a training oppurtunity or a special animation. With debugged Tamas try to see what the animations look like with different Tamas... You'd be surprised at the animations you never see unless you constantly watch the screen, such as wearing a backpack, taking a bath, singing and dancing, etc.
If you want your Tama to be back to normal again, just open it back and erase the pencil marks you made on the Debug button. Hope this helps! <_<