Hi Devin,
There are 2 ways you can do this in WEBENCH.
1) You can create a custom output cap with quantities. To create a custom output cap follow these steps.
a. Click on the BOM
b. Click “Select Alternate Part” for Cout
c. On the bottom left you will see “Create a Custom Part” button.
d. Fill in the capacitor parameters to use and click “Save changes”. This will apply the custom cap directly to your design.
2) If the device you are using has “Sim” supported in WEBENCH, then “Edit” schematic option will be available. “Edit” feature allows you to make changes to your schematic. In this case you can add more caps in parallel to your output cap and perform simulations.
Please note, if you are using option 1 above then changing the capacitor will update all OpVal calculations (including efficiency). With option 2 you can analyze the simulation results but efficiency will not be updated.
Thanks,
--Abhishek Gupta