Here's one way -
Add another variable to be used as a counter - increment when the other counter increments by >26. Use a case statement or a procedure to build your sting.
Demo code below loops through 78 x to build string to ZZZ
Dim intCharCode, intLetterCount As Integer
Dim i As Integer ' used to loop for demo...
intCharCode = 65 ' Set to "A"...
intLetterCount = 1
' Loop through this code 78 times so we can build three up to three chars
For i = 1 To 78
' Call the BuildString function passing the character count and the Character Code
Debug.Print BuildString(intLetterCount, Chr$(intCharCode))
' If the character code is 91 then reset to 65
' and increment the intLetterCount variable by one...
If intCharCode = 90 Then
intCharCode = 65
intLetterCount = intLetterCount + 1
' Increment the character code...
intCharCode = intCharCode + 1
Public Function BuildString(intLetterCount As Integer, strChar As String) As String
Dim i As Integer
Dim strTemp As String
' Loop and build the string...
For i = 1 To intLetterCount
strTemp = strTemp & strChar