1. You cannot.
2. That shouldn't be necessary; if you know your nick and password, just identify yourself with NickServ as you usually would:
Code:
/msg NickServ IDENTIFY [B][U]password[/U][/B]
The only reason you'd need to re-register your nick would be inactivity. If it has been a while since you last used your nick, it may have been dropped by NickServ and possibly taken by someone else.
You can check whether a nick is registered or not by using the following command:
Code:
/msg NickServ STATUS [B][U]nickname[/U][/B]
which will return a status code. If it returns "0" or "1" you're all right, but "2" or "3" will mean that someone else has claimed it.
Now, on a Mac,
→Colloquy is probably going to be
your best bet. I won't bother talking about the others because they're text-based, lacking full GUIs and thence mouse support--which you'll want if you're a fan of HydraIRC.