Technically you're not suppose to because it's a Fixed Dialog and its form members aren't exposed for changes.

Although, if you're mad scientist (like myself), it's possible to use the Creation Filter to grab the MessageBoxUIElement. From this element you can grab the form's reference and create an instance of WinFormManager to use with the WinMessageBox. Additionally, you'll have to tweak that form's height by roughly an extra 45 pixels to its height because the form manager shrunk it a bit.

#region IUIElementCreationFilter Members
public void AfterCreateChildElements(UIElement parent)
{
    
}
public bool BeforeCreateChildElements(UIElement parent)
{
    if (parent is MessageBoxUIElement)
    {
        if (!wasAdded)
        {
            Form messagebox_form = ((MessageBoxUIElement)parent).Control.Parent as Form;
            UltraFormManager messagebox_manager = new UltraFormManager();
            messagebox_manager.Form = messagebox_form;
            messagebox_form.Height += 40;
            wasAdded = true;
        }
    }
    return false;
}
#endregion

Below I'll paste the url to the documentation that shows how to get the creation filter set up and then you can copy the code from the above snippet to give it a shot.

http://help.infragistics.com/NetAdvantage/WinForms/2010.3/CLR2.0/?page=Win_Creation_Filter.html

[Edit - Below is a screen shot of the message box I created with WinFormManager and used the black office 2007 color scheme]