A simple recursive function like this will work for anything up to 219599 seconds (thats 60 hours, 59 minutes, 59 seconds), though you will have to add some formatting to get leading zeros:
Code:
Public Function SecsToSerial(x)
If x > 60 Then
SecsToSerial = SecsToSerial(x \ 60) & ":" & x Mod 60
Else
SecsToSerial = x
End If
End Function