You'll have to create a VBA macro to do this! The trick will be where you wish to keep your last generated serial number.
One way would be to keep it in a hidden file.
Set Word = CreateObject("Word.Application")
Word.Visible = TRUE
Word.Documents.Open("C:\LastGenNo.doc")
Now I suppose you could actually store the number inside the file itself. But it might be easier if you simply create a user-defined property.
Word.UserProperties.Add("LastGeneratedNumber", olNumber)
You'd want to then increment the value everytime, you accessed the value, then return the new value
Word.UserProperties("LastGeneratedNumber").Value
I've given you most of the pieces of the puzzle to put this together. With a little bit of VBA coding you could put these together into a macro that can act as an incrementer.
There are other places you can store the number. One interesting one would be in the ContactItem object. That would allow you to create unique numbers but for the givenContact. Go into the VBA Editor (Alt+F11) and check out the object browser for ideas.
Good luck!
Steph