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 Scan to Salesforce tab, then the wrench icon, open Admin Settings and find Field setting.
Select the object and record type, then, select “Enter a fixed value” at Card field and set Fixed value for each custom field.
Also, if you enable ”Allow users to overwrite values for fields,” users can individually edit custom field values when transferring business card data. See here for more information.