Manual confirmation of user emails is not available as a standard feature, because it is not a legitimate practice. The email account owner absolutely has to confirm their email from their inbox. As an admin, you can resend confirmation emails, setup transaction email server to insure better deliverability and you can directly contact the email hosted to assist them with the confirmation process. 

There are situations when the email owner is known to the site operator, or the site operator may be receiving separate letters expressly confirming one's email ownership. In such cases the best thing to do is to resend a confirmation letter and send a personal note to the email owner asking them to confirm via a confirmation email. 

Opting-in emails manually may be illegal. Please, refer to https://www.ftc.gov/tips-advice/business-center/guidance/can-spam-act-compliance-guide-business - you may be fined for over $42,000 for EVERY email violation, and unless you have evidence of expressed record of email owner opting-in via Confirmation link from within their email, you may not be able to prove that your emails are sent legitimately.

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Having said that, I agree that there may be situations when for some reason manual email confirmation is useful - for example for testing purposes for admins, allowing to create multiple test accounts quickly. So, we have added this feature in v11 and already implemented ( https://github.com/unaio/una/issues/2499 ) in CI, but I reiterate that it is not intended for and is not recommended for manually confirming standard users emails.

Dolphin had this feature and I have used it when members were not getting the emails and contacted the site.  Spam mail is not the same as an email confirmation; Andrew, don't scare members with this misinformation.  Perhaps someone can do a hack or a module/app to add this feature.