The form below is a very basic one. Chances are you will need something more comprehensive.
CASH EQUIVALENT FOR UNAVAILABLE STOCKS
As codicil and amendment to my will, dated ___________________, and witnessed by
______________________________, __________________________________, and ______________________________, I, ______________________________ declare the following:
In the event that the shares of stock in ____________________________________ corporation, bequeathed to ____________________________________ in my will, are not part of my estate at the time of my death, I bequeath instead to _________________________________ a sum of money equal to the market value of those shares at the time of my death.
These forms are provided AS IS. They may not be any good. Even if they are good in one jurisdiction, they may not work in another. And the facts of your situation may make these forms inappropriate for you. They are for informational purposes only, and you should consult an attorney before using them.