Pass a Variable into the Web.config

Hi ,
i would like to pass the variable into the web.config, isit possible?
<add key="Mail.Host" value="smtp.wdc.com"/> <add key="Mail.EnableSsl" value="false"/> <add key="Mail.UserName" value=""/> <add key="Mail.Password" value=""/> <add key="Mail.CCMail" value="111@abc.com, 222@abc.com"/> it is because my Mail.CCMail will always change. how to pass the variable into it.
i will get the email cc list from store procedure.