How to union dbase files into 1 query and also a custom column

[kana]

Dear All Master

With this I want the UNION database to be 1 query in the power query. I already used union in the connection string but I can't custom the columns for each dbf table. What I want as follows:

1. in the BOJ folder with the database name IFG.DBF I want to custom the column with the name DEPT and the contents of the column is "BOJ", in the M18 folder with the database name IFG.DBF I want to custom the column with the name DEPT and the contents of the column are "M18", in the MD2 folder with the database name IFG.DBF I want to custom the column with the name DEPT and the contents of the column are "MD2".
2. Can I union to the first point with a different DBF database (STF.DBF in each folder)
3. I want a function of the M code language to speed up database loading because database records are in the hundreds of thousands.
4. I hereby attach the sample file link: https://drive.google.com/open?id=1wyCZuaAUBeXDVksp7Wt1St1ITEpciIk_

let
    Source = Odbc.Query("dsn=dBASE Files", "Select * From C:\Test\mytest\m18\IFG.DBF Union Select * From C:\Test\mytest\boj\IFG.DBF Union Select * From C:\Test\mytest\md2\IFG.DBF#(lf)"")")
in
    Source