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I suppose the use case is only allowing people from a certain organization (e.g. college or company) to self-register an account. This can be done sideways by requiring admin approval for accounts or writing a script which runs periodically and blocks accounts with emails coming from other domains. It's not a security / anti-spam feature, it's a business policy feature. As such it has no place in Admin Tools.

Excluding (blocking) some known spammer domains is a matter of spam prevention. If I know that scammyspammer.eu is a nefarious operation which employs cheap labor to register to sites and post comment spam then it makes sense to block user registration from that domain name outright.

On the other hand, how the heck is it an anti-spam measure if I only let people from goodguys.eu register a user account? What about people with an email from honestgentlemen.co.uk? Or polykalapaidia.gr? Adding dozens or hundreds of domains in a whitelist sounds like you need to think about your anti-spam policy a little more.

The only valid domain white-listing has NOTHING to do with security and EVERYTHING to do with a naive, dangerous and bad implementation of business policy. If you think really hard about it you will see that in all those cases you thought whitelisting makes sense you are either confused (and better off with a blacklist of spammy domains) or you were looking for a CRM or a controlled registration system (think about JomSocial, Community Builder or even subscription software which only allows you to subscribe if you use a coupon code).

However, I will assume you have a valid use case. Let's hear what is your use case. Why do you need this feature?