[KevinH]

You can use Sigil's Spellcheck Editor Tool to add words to a user dictionary/wordlist you specify.
Check out the Sigil User Guide for details.

If these words are in a language recognized by a an official language code you may want to consider wrapping them in span tags with the appropriate lang attribute set.

See the attached image and the Add to Dictionary button. There is a pulldown to select a wordlist to add to. I recommend creating one specific to your epub via Sigil Preferences, then selecting it with the pulldown in the Spellcheck Editor dialog instead of just the "default".