Additional external dictionaries are not user-addable (edit: false; see pazos' reply), but they are trivial to add in the code (if approved). I submitted a request on the Github repository for you.
Please look into user patches. This one for a custom word selection menu for example:
https://github.com/koreader/koreader/issues/13024
Or this one for a custom sepia background image (only for reflowable documents):
https://github.com/koreader/koreader/issues/12420
(Note that user patches don't work on the F-Droid version.)
Not sure what you mean by no scrolling mode? Continuous mode is scrolling, it's just a little different UX than what you're used to from other Android apps.