Without knowing how they store the data, it can't be done. And they aren't likely to store the data in any sort of way that makes it at all easy to read/write manually.
Even if you do know, while not entirely impossible (ActiveX will let you write, and you can probably read by putting the files in an object tag and reading that) it is highly infeasible. Like making a jpeg in Notepad, only harder.
There are probably fairly easy ways to do it in several server-side languages, but Javascript is completely the wrong tool.