External ADC and FIFO
A simpler solution that I usually employ is just to modify the signal level using an analog amplifier, in that way you will be able to use the internal adc. A very simple operational amplifier circuit can convert from -10:+10 to 0:3.6 a very common configuration is shown here: http://www.electronics-tutorials.ws/opamp/opamp_5.html

I believe this is cheaper and simpler than using an external adc.