Taylor Harbin:
I was in a similar situation with a novella that I have been wanting to expand into a novel.
The novella is complete in itself, and I was afraid of breaking it if I expanded it, but lately, I have been realizing that there are possible ways.
One thing I did, is that I took two fairly-major characters and delved into their history. One character is an old wise woman, and the new chapters will explain how she became so wise, and how she overcame many difficult obstacles to arrive at the place where she currently is. A second character is a cruel man, and head of the guards at the palace -- I want to explore why he became so vicious.
So in a nutshell, that is the direction I decided to take: explore the history of these characters, to explain why they became the way the are, and how they happen to be where they are; their impetus, if you please.
I also wanted to add a short, but powerful, chapter, all by itself, that explains why the Emperor became the way he is -- just a single chapter where he has an epiphany that changes the course of his life, the course of his kingdom, and the course of my novel.
My other novel is 150,000 words, after shaving it down from 160,000 words -- so I have no problem writing longer pieces. It's shaving it down, that is the issue for me.
Anyway, consider the idea of extra chapters to explain secondary characters.
Perhaps that might help.
Best of luck to you!