C# - Adding Different E-mail's In Web.config At Run-time?

I'd like to know if there is an automated method to add e-mail settings to web.config.

There is already an e-mail set, and it's currently like this:

<system.net>
<mailSettings>
<smtp deliveryMethod="Network">
<network host="0.0.0.0" port="25" userName="whatever@whatever.com.br" password="stackoverflow" />
</smtp>
</mailSettings>
</system.net>

So what I want is:

1 - How do I add a new e-mail in there set by textboxes in the webform ?

2 - How do I differentiate which one I want to gather at the time ? could you give me a simple C# example on how to get the configs in one of the two ?