How can I export/import contact form data?

Because Contact Form 7 stores its contact form data as a custom post (post type: wpcf7_contact_form), you can export and import form data via Tools > Export and Tools > Import in the WordPress admin screen.

In the Export menu, choose Contact Forms if you want to export contact form data only. You can also choose All content (this includes contact form data).

Just another contact form plugin for WordPress. Simple but flexible.