On webapps, the storage activity does not allow you to exclude fields, by default it just updates all data submitted from the front end forms, the way to get around this is to use javascript on both front end (to assign field data to variables) then on back end to manually handle updating the fields you wish to have control over for.
It would be very beneficial for an enhacement to be made to this activity, which allows the developer to uncheck fields to be excluded.