Hi,
If you want to do that, you can edit the file product / cart.php via the menu Display>Views to add your captcha, and develop a plugin of the group "hikashop", implementing the "onBeforeCartSave" trigger to approve / cancel the cart saving based on the captcha solution provided by the customer.
Note however that I don't see the point of doing that.
Usually, you only add a captcha to forms where abusing its use can be detrimental to you. For example, on a contact form, abusing it could lead to your website sending many emails, potentially leading to your server being flagged as a spammer.
Or on the registration form, this could lead to fake user accounts being created and your server sending emails to unauthorized email addresses leading to your server being flagged as a spammer, or it could lead to the user accounts posting spam messages to other websites on a comment system, or a forum system you have on your website.
But on the cart module, submitting the form of the cart module doesn't do anything besides refreshing the cart data in the database. So I would recommend to not bother with this development.