This is a trick we used to do a-lot more back in the days before the 22 fret strat became more the norm.
The hard part is matching the finish on a maple neck. On a rosewood fingerboard you can hide it even better.
|Here is a basic 21 fret vintage style Strat neck.|
|Removing the 21st fret and adding the wood for the extra fret.|
|Here is the finished 22nd fret addition.|