Попробуйте
Sub test()
Dim vDB, vR()
Dim A, B, C
Dim i As Long, r As Long, n As Long
A = "A"
B = "B"
C = "C"
vDB = Range("a1").CurrentRegion
r = UBound(vDB, 1)
n = 1
ReDim Preserve vR(1 To n)
vR(1) = vDB(1, 1)
For i = 2 To r
If vDB(i - 1, 1) = B And vDB(i, 1) = A Then
n = n + 1
ReDim Preserve vR(1 To n)
vR(n) = C
End If
n = n + 1
ReDim Preserve vR(1 To n)
vR(n) = vDB(i, 1)
Next i
If vR(n) = B Then
n = n + 1
ReDim Preserve vR(1 To n)
vR(n) = C
End If
Range("c1").CurrentRegion.Clear
Range("c1").Resize(n, 1) = WorksheetFunction.Transpose(vR)
End Sub
Если вы хотите несколько столбцов, то
Sub test2()
Dim vDB, vR(), vS()
Dim A, B, C
Dim i As Long, r As Long, n As Long
Dim col As Integer
Dim Ws As Worksheet
A = "A"
B = "B"
C = "C"
vDB = Range("a1").CurrentRegion
r = UBound(vDB, 1)
col = UBound(vDB, 2)
n = 1
ReDim Preserve vR(1 To col, 1 To n)
For j = 1 To col
vR(j, n) = vDB(1, j)
Next j
For i = 2 To r
If vDB(i - 1, 1) = B And vDB(i, 1) = A Then
n = n + 1
ReDim Preserve vR(1 To col, 1 To n)
vR(1, n) = C
End If
n = n + 1
ReDim Preserve vR(1 To col, 1 To n)
For j = 1 To col
vR(j, n) = vDB(i, j)
Next j
Next i
If vR(1, n) = B Then
n = n + 1
ReDim Preserve vR(1 To col, 1 To n)
vR(1, n) = C
End If
Set Ws = Sheets.Add 'Sheets("Result")
With Ws
.Range("a1").CurrentRegion.Clear
.Range("a1").Resize(n, col) = WorksheetFunction.Transpose(vR)
End With
End Sub