I do not use Processor Expert, so I have no example for you, but the way I accomplish what you are trying to do is the following:
1) configure a PDB or other timer to periodically trigger the ADC (described in Chapter 19 of the KQRUG)
2) configure the DMA to be triggered by the ADC conversion completion (using the DMA's MUX SOURCE setting)
I am not sure whether there is a way to configure the device so that the DMA triggers the ADC conversion, as you described.
Be careful to turn off the timer triggering the ADC when the DMA is disabled, as the ADC can have a fault if its result register is not read before the ADC completes another conversion.
KQRUG: http://cache.freescale.com/files/32bit/doc/quick_ref_guide/KQRUG.pdf