One can not create a search field directly while searching and reviewing in DISCO. However, you can request for a custom field to be added to your metadata by contacting DISCO support. One of our Technical Success Associates will be able to create and apply an overlay into your database which will add your custom search field into the metadata. By ingesting this overlay your new custom search field also becomes searchable in the database!
However, if you have a matter created on or after 12/06/2016, you may create custom searchable fields. Please read 12/06/2016: Fields for more detail.
Of course let DISCO support know if you have any questions!