IC Layout - NMOS Substrate Connection
If you want a different substrate voltage in p-substrate process:  

1) Create a n-well in the p-substrate and connect this new n-well to VDD. The depletion layer between the n-well and the p-substrate does the isolation job.
2) Inside the n-well, create a p-well and connect this to the body-bias voltage.
3) Inside the p-well you can create  n-mos transistors whose substrate voltage is different from the rest of the chip.  

n-mos ---> p-well(VSS2) ---> n-well(VDD) ---> p-substrate(VSS1)