How to include Server side error message with validator control in validation summary

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ASP.NET

Hi How to include server side validation error(Code behind) message and validator control(aspx Page) message in validation summary

Aspx Page:

 <asp:label id="lblerror" runat="server">
          <asp:validationsummary id="valsum" runat="server" validationgroup="settingsProduct"\>

<asp:textbox id="username" runat="server"\>
<asp:Requirefieldvalidator id="Requsername" controltovalidate="username" validationgroup="settingsProduct">
<asp:button id="btnclick" onclick="btnclick_click" runat="server"\>

C#
---
Now i check username is available in db and display error message in label

Public void btnclick_click(object 0,Eventargs e)
{
    if(username)
   {
     lblerror.text="User ID is not found";
    }    
}

My Question :
Now display separate label error message and validation summary.
Can we combine both error message?
Plz help me...I have no idea...