Tinycore assumes cde/optional = cd/dvd = read only - this is why you cannot add extensions to cde/optional
Tinycore also assumes iso = read only - this is why you should remaster if you wish to add extensions to it
You are, of course, welcome to try whatever you like, but it will not be supported officially.
As already suggested, why not install tinycore to your sd card and boot directly from it rather than from an iso?