You can modify the comment form. WordPress provides various hooks. See comment_form().
Actions
- comment_form_comments_closed
- comment_form_before
- comment_form_must_log_in_after
- comment_form_top
- comment_form_logged_in_after
- comment_form_before_fields
- comment_form_after_fields
- comment_form
- comment_form_after
Filters
- comment_form_default_fields
- the_permalink
- comment_form_defaults
- comment_form_logged_in
- comment_form_fields
- comment_form_field_comment
- comment_form_field_{$name}
- comment_form_submit_button
- comment_form_submit_field