I found a way of creating custom wordlists, using aspell on the command line.
So for those who want to know here is a quick how to:
1) Ensure you have a base language dictionary (i.e. en for english).
2) Create a wordlist file (a single word per line (alpha characters only))
3) use aspell to create the .rws
aspell --lang=en create master /path_to_dictionary/en_newdictionary.rws < wordlist
That will create your wordlist rws file.
4) Create a .multi text file in the same dir as your rws file, so aspell knows what to reference:
example:
en_newdictionary.multi
add en_newdictionary.rws
All done, now to exclusively use your wordlist use:
$pspell_link = pspell_new("en_newdictionary");
All this is in the Aspell manual chapter 5.