You could modify the netlist by hand to create the mismatch in the devices you want and re-run the simulation.
Alternatively, you can copy the schematic to a different view (say schematic_mismatch), modify the devices to be mismatched appropriately, and then use hierarchy editor to run the simulation using the same test bench by pointing to schematic_mismatch instead of the schematic view for that cell.
In both the above cases, you can simulate at most a few cases since everything is being done by hand, and it gets cumbersome if your design is large.
If your foundry provides mismatch models, then you also may be able to use the Monte Carlo tool (click on Tools --> Monte Carlo...) in ADE. The procedure for how to modify the model file to include mismatch parameters using a statistics block is available in a white paper on
www.cadence.com/whitepapersThanks,
Setu