Files from a folder - adding columns

Here is an example that combines all .csv files in C:\temp.  It assumes that the files have the same structure, and provides dummy column headers - you may want to provide your own.  It also doesn't strip out potentially existing column headers inside the files - you can filter those out if needed, or modify the code for the AddColumn accordingly.

let
    Source = Folder.Files("C:\Temp"),
    #"Filtered Rows" = Table.SelectRows(Source, each ([Extension] = ".csv")),
    #"Added Custom" = Table.AddColumn(#"Filtered Rows", "Custom", each Csv.Document([Content])),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Added Custom", "Custom", {"Column1", "Column2", "Column3", "Column4", "Column5", "Column6"}, {"Column1", "Column2", "Column3", "Column4", "Column5", "Column6"})
in
    #"Expanded Custom"