User Controls :: Programmatically Add Update Dynamic Roles In Location Tag Of Web Config

I want to add a new rule in my web.config file programmatically,

ex: I have a textbox to add location path I have a combobox (with items admin and user) to add roles. ex: user input location path = textbox roles = combobox (items admin and user) the save button, when click the save button the results go to the web config :

<location path="Report/ManageReport.aspx">
<system.web>
<authorization>
<allow roles="ADMIN" />
<deny users="*" />
</authorization>
</system.web>
</location>

So far my code as below: but I was confused to apply its

protected void AddRoleRule(string location, string selectedrole)
{
Configuration config = WebConfigurationManager.OpenWebConfiguration(Server.MapPath("~/Web.config"));
XmlDocument xDoc = new XmlDocument();
xDoc.Load(config.FilePath);
//if the rule exists update the rule

[Code] .....