After a bit more thought, I realize the Blender addon project could be simplified by keeping the addon as is but have the selection choose a folder of DAT files, then have the addon run the code to create the Final.DAT from all the numbered DAT files in the folder, and, as a not required bonus, place that Final.DAT file into the selection field. Everything else could stay exactly as it is.
A quick test shows the current addon can select a folder and fill the selection field with that folder path, even with no file selected. So, a separate button could be added that will call the Final.DAT creation code, using the folder path in the selection field as the working folder for the code. Then, the selection field can be used again to select that Final.DAT, or one could type Final.DAT into the selection field, or the code could append the folder path with the filename 'Final.DAT'.
A quick test shows the current addon can select a folder and fill the selection field with that folder path, even with no file selected. So, a separate button could be added that will call the Final.DAT creation code, using the folder path in the selection field as the working folder for the code. Then, the selection field can be used again to select that Final.DAT, or one could type Final.DAT into the selection field, or the code could append the folder path with the filename 'Final.DAT'.