In later versions of MWG all list types have the "append from file" option. Unfortunatly I cannot find any docs which explain what the list format should look.
So the question is, where can I find some docs which cover, for each list type, exactly how the "append from file" input list should look like ?
The imported file can be a flat text file. When you import or append to list, a wizard should come up asking to identify which column of the file and the list type.
Once the column is selected, the list contains those entries.
Some of this depends on version, you may be on an older one that doesn't have the import wizard. If so, you may have to use the XML format. Export a similarly typed list and look at it's structure and duplicate.