hCaptcha founder here. You should be able to do this with some backend code and a small client side change. Happy to help make that happen. What framework are you using?
Unfortunately, all (ok, the only) the hcaptcha plugins available basically just let you enter a sitekey and nothing else/no other configuration possible.
Simply don't add hcaptcha to the page for approved accounts or ignore absence of a solved captcha on the server in submitted data. If you can't whitelist approved accounts, then you have a problem.