Its another easiest thing can be added to drupal base site without giving extra effort . Comment field can be added to any type of content type of Drupal . Here we are sharing all steps to add Comment field . here it goes .
i) Navigate to the Content types administration page (Administer > Structure > Content types or http://example.com/admin/structure/types).
2) Find the content type for which you'd like to add fields on the comment form, and click "Manage fields" next to it.
3) Click the "Comment fields" tab.
4) In the "Add new field" section, enter a human readable "Label" for the field.
5) In the "Name" field, enter the machine name for the field. (You can only use numbers, lower-case letters, and underscores.)
6) In the "Select a field type" dropdown list, select one of the following types:
Long text and summary
7) In the "Widget" dropdown, select a widget. The options vary depending on the field type chosen.
8) Click "Save".