I'd like to make flyspell aware of certain words I use frequently--like foreign proper names, etc. But I don't want flyspell to think that this list is a complete dictionary; rather I'd like it to take the union of this list and its default dictionary.
How can I do this?
As I understand it, flyspell is the on-the-fly minor mode that highlights misspellings. It uses ispell (or whatever you've set as ispell-program-name) to do the actual spelling.
When I correct words in flyspell, I can add them to a personal dictionary. It's one of the options.
Are you asking for a way to pre-populate your personal dictionary? (In which case, we need to know what spell checker you use, ispell? aspell?)
Are you asking for a way to switch between personal dictionaries? If so,
(setq ispell-personal-dictionary "my-personal-dict")
See also Spelling in the emacs manual. (And C-h v ispell-personal-dictionary) Of course, that assumes a correctly formatted dictionary. I think personal dictionaries are essentially just word lists, but I don't know whether they have to be sorted, or maybe there are other requirements on the format.