VIM spell files seem to be in some binary format. Is it possible (and easy) to convert one back to a plain text word list?
Converting .spl spell files
spell checking
Yes, you can use :spelldump.:spelld[ump]            Open a new window and fill it with all currently valid                        words.  Compound words are not included.                        Note: For some languages the result may be enormous,                        causing Vim to run out of memory.Vim will use the language file that you set with the 'spelllang' option.