Please note you need an admin account to do this.
Yes, you can set fixed values to the custom fields in your Salesforce organization.
Go to the Scan to Salesforce tab, then the gear icon. Open Admin Settings and click Field settings. Select the object and record type, then select Enter a fixed value for the Card field and set a fixed value for each custom field.
Also, if you enable Allow users to overwrite values for fields, in Transfer settings, users can individually edit custom field values when transferring business card data. See here for more information.