Unfortunately there is no such utility available that can update any additional property to all the pages under a specific folder.
How frequently you need this property to be updated?
If this is going to be one time update, I will suggest to go with a servlet which will just take the source folder path as a parameter and iterate over all the pages available under the folder. If any page found, try to look for the property you are trying to add and if it does not exist then add it. If it exists, then skip it.
If it's going to be done more frequently, then you can go by authoring by adding another field into the page property dialog and then using the inheritanceValueMap, in this way you can author only on the parent page and all the child page will inherit the same property. If you want to change in future you can change at one place, or if you want to override at a child page level also can be done.