Hi,

 This is possible. It requires that you create a class that inherit that page, and extends it.

namespace EPiServer.Templates.RelatePlus.Templates.RelatePlus.AdminExtension {
     public class EditUserPage : EPiServer.Community.Web.Administration.EditUserPage
     {
         protected System.Web.UI.WebControls.TextBox txtExtraBox;
         protected override void  OnInit(EventArgs e)
         {
             buttonSave.Click += new EventHandler(buttonSave_Click);
             base.OnInit(e);
         }
         void buttonSave_Click(object sender, EventArgs e)
         {
             if (Page.IsValid)
             {
                 // Save the text in the added field txtExtraBox
                 string text = txtExtraBox.Text;
             }
         }
     } 
}

Then, you open up EPiServerCommunity\Security\EditUser.aspx, and change the @Page directive so that the page inherits the class above instead of the default implementation:

@Page ... Inherits="EPiServer.Templates.RelatePlus.Templates.RelatePlus.AdminExtension.EditUserPage" 

And also in this file, add the user control that is handled by your class:

<asp:TextBox ID="txtExtraBox" runat="server"></asp:TextBox>

This should be it!

/Kristoffer