Is there a way to include Mac OS X built-in dictionary/thesaurus in TeXStudio?
TeXstudio can only handle Hunspell dictionaries (as they are used in OpenOffice / LibreOffice). A converter/export from OSX dictionary to Hunspell would be the way to go. Unfortunately, I'm not aware of such a tool, but I'm not an expert there.
By default, the TeXworks resources folder should be located inside the Library folder of your Home (user) folder. This is automatically created the first time you run TeXworks, and some default resources are copied there. There should be a dictionaries subdirectory, but it is initially empty. Place the *.dic and *.aff files there, and then re-start TeXworks.
If your dictionary files are wrongly formatted I'm sure you can use some nifty grep script to fix it.
I've found LaTeX Editor hosts downloads for various different dictionaries and they seem to be quite comprehensive.