Is it possible to override the localization files provided (or not provided) by a 3rd party plugin? A plugin that I'm using now doesn't have any language files but I want to avoid having a future update to the plugin overwrite the translation files that I've created.
From what you are saying you have translated your plugin and now you want to be sure the future update won' be overwritten. So to avoid that happening you can always put your po/mo translation files (if provided) in global languages folder that is located on your server in wp-content/languages/plugins folder.
If plugin have language file name for example someplugin.pot, from there you can generate someplugin-en_US.po and someplugin-en_US.mo files and put them in folder I mentioned before. Note that I put language code en_US just as an example you will add your language code.
Or you can just not update the plugin (but not recommended). There is also a plugin that disables plugin from auto update here.
Also for all kind of translations I would recommend using you LOCO TRANSLATE which you can use and automatically put your po/mo files generated from plugin .pot to global languages folder.
I hope this helps.