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For convert capatcha to recaptca in your site, you most modify mode in array $config in /class/captcha/config.php to
'mode' => 'recaptcha',
Add google key in /class/captcha/config.recaptcha.php 'private_key' and 'public_key'.
Now your security code is in recaptcha mode.
For add recaptcha in other module i see as work module profile. register.php and /include/forms.php files