Jose
if you want to sample ADC0_SE7b rather than ADC0_SE7a (which your code is doing) set the MUX selection bit to B in the ADC0_CFG2 register.
That is, add
ADC0_CFG2 |= 0x10;
after
ADC0_SC1A = ADC_SC1_ADCH(31);
For ADC0_SE6b the same MUX bit must be set (there is no ADC0_SE6a input on your device).
Regards
Mark
http://www.utasker.com/kinetis.html
P.S: The uTasker project also includes DMA driven ADC/DAC drivers for KL devices!