Function HexAdd(ByVal strNum1, ByVal strNum2)

size1 = Len(strNum1)

size2 = Len(strNum2)

If (size1 <> size2) Then

If (size1 > size2) Then

strNum2 = String(size1 - size2, "0") + strNum2

size2 = size1

Else

strNum1 = String(size2 - size1, "0") + strNum1

size1 = size2

End If

End If

' Begin adding

Dim i

Dim ret

Dim carry

Dim sum

ret = ""

i = CDbl(0)

carry = 0

For i = 1 To size1 '199999999999999999999998

a = "&H" & CStr(Mid(strNum1, size1 - i + 1, 1))

b = "&H" & CStr(Mid(strNum2, size1 - i + 1, 1))

sum = CInt(a) + CInt(b) + carry

ret = Hex(CStr(sum Mod 16)) & ret

carry = Fix(sum / 16)

Next

If (carry > 0) Then

ret = CStr(Hex(carry)) & ret

End If

HexAdd = ret

End Function

Call it like this:

ret = HexAdd("FFFFF", "123456789ABCDEF")