Yes, it's possible to reuse an existing privacy checkbox that is already present in your form.
If your form already includes a checkbox for the privacy policy that is not generated by the GDPR component, you can just reuse it instead of adding a new one and at the same time take advantage of the tracking system provided by the GDPR component and based on the 'Logs ofuser consents'.

To link an existing checkbox with the GDPR component, it's enough to specify a CSS selector to associate that checkbox using the following parameter, so that when a user will select it, the tracking will be visible in the 'Logs of user consents' as if it was a native GDPR checkbox.