How can I conditionally prevent a column from being updated in SQLite?

I have a table with CREATED and MODIFIED columns. I only want to insert the CREATED value once and have it be thereafter immutable. I know how to do this in a tedious way (write a "DoesRecordExist()" method and then alter the query and number of query parameters based on that), but surely there is a slicker way to accomplish this. After all, this has to be a common requirement (a "database pattern" if you will).

My code is this:

public void InsertUserSiteRecord(UserSite us)
{
    using (SQLiteConnection conn = new SQLiteConnection(HHSUtils.GetDBConnection()))
    {
        conn.Open();
        using (SQLiteCommand cmd = new SQLiteCommand(conn))
        {
            cmd.CommandText =
                String.Format(
                    @"INSERT INTO UserSite (SiteNum, SerialNum, UserName, Created, Modified) 
                                VALUES (@SiteNum, @SerialNum, @UserName, @Created, @Modified)");
            cmd.Parameters.Add(new SQLiteParameter("SiteNum", us.SiteNum));
            cmd.Parameters.Add(new SQLiteParameter("SerialNum", us.SerialNum));
            cmd.Parameters.Add(new SQLiteParameter("UserName", us.UserName));
            cmd.Parameters.Add(new SQLiteParameter("Created", us.Created));
            cmd.Parameters.Add(new SQLiteParameter("Modified", us.Modified));

            cmd.ExecuteNonQuery();
        }
        conn.Close();
    }
}

...and I want to avoid having to do something like this:

public void InsertUserSiteRecord(UserSite us)
{
    using (SQLiteConnection conn = new SQLiteConnection(HHSUtils.GetDBConnection()))
    {
        conn.Open();
        using (SQLiteCommand cmd = new SQLiteCommand(conn))
        {
            if (!RecordExists(us.SiteNum, us.SerialNum, us.UserName))
            {
                cmd.CommandText =
                    String.Format(
                        @"INSERT INTO UserSite (SiteNum, SerialNum, UserName, Created, Modified) 
                                    VALUES (@SiteNum, @SerialNum, @UserName, @Created, @Modified)");
            else
            {
                cmd.CommandText =
                    String.Format(
                        @"INSERT INTO UserSite (SiteNum, SerialNum, UserName, Modified) 
                                    VALUES (@SiteNum, @SerialNum, @UserName, @Modified)");
            }
            cmd.Parameters.Add(new SQLiteParameter("SiteNum", us.SiteNum));
            cmd.Parameters.Add(new SQLiteParameter("SerialNum", us.SerialNum));
            cmd.Parameters.Add(new SQLiteParameter("UserName", us.UserName));
            if (!RecordExists(us.SiteNum, us.SerialNum, us.UserName))
            {
                cmd.Parameters.Add(new SQLiteParameter("Created", us.Created));
            }
            cmd.Parameters.Add(new SQLiteParameter("Modified", us.Modified));

            cmd.ExecuteNonQuery();
        }
        conn.Close();
    }
}

private bool RecordExists(String SiteNum, String SerialNum, String UserId)
{
    // query the table to see if those three values exist in any record
}

Is there a SQL[ite] construct that is something like:

cmd.Parameters.AddOnlyIfColumnIsEmpty(new SQLiteParameter("Created", us.Created));

? Or how can this be best tackled?