just jack the 9v, assuming its a alkaline straight into your nimh pack, maybe a blocking diode to prevent back charging... no resistor required... 9v's have such a low mah rating and high internal resistance, your external resistor would just be redundant.
but like Hero points out, it is pointless, since a 9v doesn't contain enough energy to recharge those larger cells.
figure 9v * 300 mAh = 2.7 w/h ... and the aaa pack might be 7.2v * 650 mAh = 4.68 w/h - not even close!