[tbworld]

This could be done, but this is something you do not want to do unless you are a small private board. If your not careful you will release the email addresses to the internet -- it is why it is not a predefined field. With the "memberlist" there is already an option in the ACP (Admin Control Panel) to add an email icon.

This information is always available to administrators via the "User Manager (what it should be called)" ACP--> Users--> Search for Users, where you can generate any kind of list you want.

Think this through carefully as I recommend you not do this, but if you still want to move forward and you have the necessary skills, then I will point you in the right direction. It would require some template edits and a few file edits.