Here's one way:

Function Conv(Figure As String, _
   FromBase As Long, _
   ToBase As Long, _
   Optional NumberOfDigits As Long) As String
'Leo Heuser, October 1999, September 2006
'=conv(Figure,FromBase,ToBase,NumberOfDigits)
'Examples: =conv(1234,6,16,6) or =conv("45ff",16,2,32)
'If NumberOfDigits is set to 0, left out or set to fewer digits
'than are in the result, the result will be displayed without
'leading zeroes.
'The setup will convert a number from base 2-16
'to another base 2-16, but the string Digits can be
'expanded to Z, and thereby covering base 2 through 36
'If the line "Figure = UCase(Figure)" is deleted, it's possible
'to place lower case letters in Digits to cover base 2-62.
'Please keep the above text, if you pass on this routine.

Dim Digits As String
Dim ToBaseTen As Currency
Dim Dummy As Variant
Dim Counter As Long
Dim Result As String
   Conv = "Input error"
   Digits = "0123456789ABCDEF"
   If ToBase > Len(Digits) Then Exit Function
   Figure = UCase(Figure)
   For Counter = 1 To Len(Figure)
       Dummy = Mid$(Figure, Counter, 1)
       If InStr(Left$(Digits, FromBase), Dummy) = 0 Then
           Exit Function
       Else
           ToBaseTen = ToBaseTen + (InStr(Digits, Dummy) - 1) * _
               (FromBase ^ (Len(Figure) - Counter))
       End If
   Next Counter
   While ToBaseTen > 0
       Result = Mid$(Digits, ToBaseTen - Int(ToBaseTen / ToBase) * _
           ToBase + 1, 1) & Result
       ToBaseTen = Int(ToBaseTen / ToBase)
   Wend
   If NumberOfDigits = 0 Or NumberOfDigits < Len(Result) Then
       Conv = Result
   Else
       Conv = Right$(String$(NumberOfDigits - Len(Result), "0") & _
           Result, NumberOfDigits)
   End If
End Function