Hi @mm35,
Yes, you will need to define the columns in the load file so the data from the file can be loaded into the workflow temporary table. I suspect when you used the direct file load, you used the load file activity and used the automatically configured file structure and column definitions. If your existing files match that structure and column definition, you can connect your download file activity to that load file activity, just make sure to check "use the file specified in the inbound transition".
Hope this helps,
Jim