A rollover to a IRA or 401 is quite unlikely since a kiwisaver account is not a qualified account that could be transferred.
If you liquidate the kiwisaver account, that income is reportable on a US tax return since your spouse is a Green Card holder and all worldwide income must be reporte on a US tax return. There would be a credit for foreign taxes paid if taxes were due to NZ. I am assuming no tax was ever paid on the money contributed into the kiwisaver account.
