Again it will be custom coded, but you can also use the Global System Events and catch when a form is submitted, to MD5 or encrypt or whatever you want to do with the password field.
https://docs.kentico.com/display/K9/Handling+object+events
https://docs.kentico.com/display/K9/Reference+-+Global+system+events
If this is a password though, are you trying to capture users? there are options in the user registration to make them not approved and you can use ALternative forms to preset a User registration form that has the information you want (that way when they enter the password, THAT is automatically encrypted since it's part of the user's module). Just checking.