Simply add text, php or javascript to your comment form with Comment Form Text.

Comment Field Flexibility

Invite users to join your network or provide a quick link for members to change Avatars. Include a ‘Share this’ button or an advertisement. Use the comment box to creatively reach visitors and members with custom messaging.

Comment Form Text adds endless options for including custom messaging, advertisements, calls-to-action and whatever else you can dream up to the WordPress comment box.