In fact the options are editable: you cannot remove the built-in options, but you can add modified versions of them by saving the changes under a new name. (You cannot make changes and save under a pre-existing name.)
So, the easiest way to add convert options is to use the 'Copy' button and then modify the commands and titles.
For FLAC:
Select [Built-in] FLAC (compression leve 5) --> Copy --> Title: flac compression level 8
, command line: flac -o %o -8 -ignore-chunk-sizes -
--> Ok
See flac -h
in Terminal for variables. 0 is the largest file (best quality) and the fastest process, 8 is the smallest file (best compression) and slowest process.
Similarly for the other formats.