It is not possible to add attributes to the li by altering the ID. Normally you could try this
$item->ID.= '" data-anchor="'.$datamenuanchor;
but that will not work because of the esc_attr() function where the value of $id is used
$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
$output .= $indent . '<li' . $id . $class_names . '>';
I’m pretty sure that to add this data attributes to the li’s that you’ll need to create a custom nave walker.
