Только и исключительно VBA-макросом.
Например, так:
Public Sub process_data(index_range As Range, values_range As Range)
Dim values()
Dim i As Integer
Dim coll As New Collection
Dim onecell As Range
values = values_range.Value
For i = LBound(values, 1) To UBound(values, 1)
coll.Add Item:=values(i, 1), Key:=Left(values(i, 1), 1)
Next
For Each onecell In index_range
onecell.Offset(0, 1).Value = coll(onecell.Value)
Next
End Sub
Sub test()
Call process_data(ThisWorkbook.Sheets(1).Range("A1:A26"), ThisWorkbook.Sheets(1).Range("B1:B3"))
End Sub