Change references where required.
Sub Add_Sheet_To_All_Files()
Dim sPath As String
Dim Wb As Workbook
Dim sFile As String
Dim hdrArr
hdrArr = Array("Col 1", "Col 2", "Col 3", "Col 4", "Col 5", "Col 6", "Col 7", "Col 8", "Col 9", "Col 10")
sPath = "C:\Folder A\Sub Folder A\"
sFile = Dir(sPath & "*.xl*")
Application.ScreenUpdating = False
Do While sFile <> ""
Set Wb = Workbooks.Open(sPath & sFile)
Wb.Sheets.Add(After:=Wb.Sheets(Sheets.Count)).Name = "Brand_New_Sheet"
With ActiveSheet.Cells(1, 1).Resize(, 10)
.Value = hdrArr
.Offset(1).Formula = "=R[4]C+R[4]C[3]"
End With
Wb.Close SaveChanges:=True
sFile = Dir
Loop
Application.ScreenUpdating = True
End Sub