It's a little more involved than that. For example, when you define a "Host Object", will it accept an IPv6 address format? Does Nagios support IPv6 MIB extensions? Does Nagios support IPv6 Native ONLY devices? etc.
There are only a handful of vendors that support SNMP management of IPv6 devices. None that I know of that support IPv6 Native devices. So, I think it's a little more involved than just underlying OS support of IPv6.