As far as I can see so far by web searching, there is no way to either add words IN BULK to the Opera spell check dictionary (which seems to be the same currently as the Chrome default dictionary) or to add a dictionary extension in the SAME language as the default dictionary. One either has to add words one by one to the spell check dictionary--which cannot be done for thousands of medical words, for example--or one can add a totally different dictionary (e.g. UK English or another language) and SWITCH to it, thereby giving up the default dictionary in the process.
Microsoft Word for years has allowed the addition of custom dictionaries that can ALL be active simultaneously. I would think this same useful feature would not be impossible for Chrome or Opera to engineer, and I think it would be very helpful to thousands of people. Many people are typing inside of health portal text boxes, for example, yet we can't have both an English spell checker and a medical spell checker active at the same time. It seems like a major oversight in Opera (those who use Chrome can at least add on an App to help, but Chrome apps cannot be used in Opera).
Unless Opera or some extension developer fixes this soon, I'm going to have to try to combine a medical dictionary and a regular English dictionary into one large file and switch to that custom "language" dictionary inside of Opera.
If anyone else has any better suggestions, please let me know!