Hi Enrique,
Unfortunately, there is not a special configuration that allows you to lower the GPIO9 consumption to 12uA. This increase in the power consumption is very likely caused by the sensor connected, and not the LPC802 itself (I recommend that you check the data sheet of your sensor to see how much current does it draw).
One thing you could attempt is to disable the GPIO9 configuration in order to lower the power consumption when you're not using the sensor, or you could also try the "deep power-down" mode of your LPC802 in order to achieve a reduction in your power consumption when you use your sensor (you can wake the LPC up periodically in order to read the sensor).
I hope this information can help you.
Ricardo Delsordo