Is there a joomla tool that registers domains... for example something like a webservice that interacts win ICANN?

I wouldn't use them though. I've tried a few, and in most cases you don't know which whois server(s) the script will query to see if a domain is available.

In many cases, there are bots running on these servers that will steal the domains you search for (if they're available).

Not worth the risk IMO.

(my two cents)