There are MANY ways of accomplishing the same thing in mIRC scripting (as in most languages).

If-Then-Else:
on *:JOIN:#: if ($nick == MrMagooba || $nick == Magooba) msg VoiceBot VOICE $nick

Tokens:
on *:JOIN:#: if ($istok(MrMagooba Magooba, $nick, 32)) msg VoiceBot VOICE $nick

Regex:
on *:JOIN:#: if ($regex(%nick,/^(Mr)?Magooba$/)) msg VoiceBot VOICE $nick

Userlevels:
on Magooba:JOIN:#: .msg VoiceBot VOICE $nick

/auser Magooba MrMagooba!*@*
/auser Magooba Magooba!*@*

or if you would prefer to use level number 523 for AutoVoice instead:

on =523:JOIN:#: .msg VoiceBot VOICE $nick

/auser 523 MrMagooba!*@*
/auser 523 Magooba!*@*