Even if the built-in spelling dictionaries were editable, it would not be a good idea to add to them your own modifications, because they could be overwritten by a system update. Much better to follow the Unix/Mac OS X paradigm of separate system, local, user domains.
You mentioned 2600 words. If you need to spell check specialised jargon (eg, medicine), try looking for OpenOffice-compatible spelling dicts (eg, OpenMedSpel for medicine). Once you get an OpenOffice extension file (.oxt), just change its file name extension to .zip, decompress it, and copy the .aff and .dic files to <~/Library/Spelling>. (No need to bother with any other files in the archive.)
For (free) off-line reference and translation, here are a few suggestions:
Pour nos amis francophones, Le Dictionnaire de l'Académie Française, 8ème édition (1935).
<http://www.etresoft.com/etreref.html>
(Yes, it is a bit old, but some things, just like a good French wine, get only better with age. The 9th edition, a work in progress, is available on-line. Larousse and Le Robert are partly available on-line; the off-line versions are not free and are not compatible with Dictionary.app.)
If you're learning German, you have your choice of BeoLingus
<http://www.tekl.de/english/BeoLingus_German-English.html>
and OpenThesaurus
<http://www.tekl.de/english/OpenThesaurus_German.html>
or dict.cc
<http://lipflip.org/articles/dictcc-dictionary-plugin>
If you're learning Chinese, both Píng guǒ
<http://www.alex-betty.com/pingguo-dictionary/>
and the Xiǎo cí diǎn set
<http://xiaocidian.com/>
are based CC-CEDICT.
Finally, as a piece of minor trivia, if you install the Server Admin Tools, you also get Apple's Information Technologies Dict (but which, unfortunately, is neither comprehensive, nor up-to-date).