Expando.cs

public static class ExpandoExt
{
    /// <summary>
    /// Add a property to a dynamic object at runtime.
    /// </summary>
    /// <example>
    /// dynamic expando = new ExpandoObject();
    /// expando.Name = "Brian";
    /// expando.Country = "USA";
    /// expando.AddProperty("Language", "English");
    /// </example>
    /// <param name="expando">The dynamic object to which the property will be added.</param>
    /// <param name="key">The name of the property to add.</param>
    /// <param name="value">The value of the property to add.</param>
    public static void AddProperty(this System.Dynamic.ExpandoObject expando, string key, object value) =>
        (expando as IDictionary<string, object>)[key] = value;
}

Usage

dynamic expando = new ExpandoObject();
expando.Name = "Brian";
expando.Country = "USA";
expando.AddProperty("Language", "English");