Hello,
Currently there are two possible workarounds for this issue:
1) You could handle the CommaneExecuted event and check if the command's name is Modify Connection Points:
private void OnDiagramCommandExecuted(object sender, CommandRoutedEventArgs e)
{
    if (e.Command.Name == CommandNames.ModifyConnectionConnectionPoints)
    {
        var connection = this.diagram.SelectedItems.OfType<IConnection>().FirstOrDefault();
        if (connection != null)
            //connection.ConnectionPoints
    }
}
2) Use custom ConnectionManipulationTool (inherit ours), override its MouseDown method and check if in this case you'll add a new Point