This is an extension rule (Ex) and is not part of the official rules. That being said, the idea of characters having a will is supported by the 1981 character record sheet.1)
If a character dies with money or items at a moneychanger, but has not written a will, the stored wealth is lost. A will is simple statement of what happens to these items upon death. It should be sent to the DM.
Wills can be updated any time the character is in a city (see settlements).