Good afternoon,
Thanks for reaching out and for providing the details of your current setup and the workaround you’ve found!
You are right that referring to the catalog number from the determination table to add the “NHMD” prefix, while effective, isn’t the most scalable or recommended solution. It bypasses the intended mechanism for formatting these fields.
The preferred and more robust approach to achieve this is indeed by editing the table format directly within the Schema Configuration for the CollectionObject table. This allows you to define how the catalogNumber field should be presented universally across Specify, including in your DwCA exports.
First, in the user tools go to the Schema Configuration and select collection object as your base table.
Then, click on the 
icon next to table format and add NHMD as a separator.
You can change the formatting in this tab and immediately in the preview below see what the formatting would entail.
Thanks!