If your autochanger has an input as well as an output connector it will be possible. The iPod adapter would have to be plugged into the changer's input, and the changer's output cable would run to the KD-G421.
This manual here from JVC shows a diagram on the middle of page 1 where an i-Pod adapter (I'm assuming you have the KS-PD100) is connected daisy-chained with an XM tuner or a DLP unit. Although it doesn't explicitly say that it would work with a CD changer, I don't think it'll be a problem as long as you have an input on the changer.
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