You can use Custom Actions feature of our product to accomplish your scenario.
Under QWP installation folder you'll find Sample Code folder that contains a working sample Visual Studio application about Custom Actions with plenty of explanations for real scenarios. Also the documentation talks in details about this feature (see section Custom Action Help).
This custom code allows you to apply your encryption method and save the SSN field encrypted. e for visualization, use the decrypt method.
If you have any issues with the custom actions please let me know,