I think what they might be trying to tell you is to try an alternate version of English dictionary. I am using the standard one for the United States and immersive is not in it, but humorous and outrageous are. Immersive is of recent coinage so I am not surprised (other forms: immersible, immersion, and immerse are in the dictionary). To change your dictionary go to settings and search for dictionary.
Unfortunately I do not have any experience with dictionary extensions so I cannot help with your actual question. You can right-click 'add to dictionary' if there are just a few of these words you commonly use, but I doubt this is good enough for your needs.
Sorry!
Andrew

Thank you all for trying! I have been using Opera since the 90s and I know this was a feature then, though I think you had to turn it on and off globally through a right-click, later it was saved in site preferences, and later still you had to go into site preferences via the right click to find it.

I suppose it was decided that this was no longer needed since there are so many browser options now that it is uncommon for a site to demand specific browsers; Back in the day it was common for web sites to demand IE or Safari even though Opera worked just fine on them...

Thanks!
Andrew

You used to be able to right-click on a page and set an individual site's preferences including to identify as another browser or mask as another browser. I can no longer find this functionality and am trying to deal with a site that keeps throwing me to an "upgrade your browser" page.

Would you please let me know where this functionality now lives or if you know it has been eliminated?

Thank You!
Andrew