Can I write about financial circumstance in the additional information section?
At a recent admissions visit, the admissions counselor said we can write pretty much anything in that section that we think is important for them to know.
Yes, you can. But think about how would it affect your admission. Is it for explaining some ostacles you have faced during high school years that have affected your grades?
No, it is just a kind of a financial aid essay that is completely irrelevant to my grades
I would not. Admissions officers read the application and their function is admissions decisions, not financial aid. If you have an unusual financial circumstance you might want to take it up with the office of financial aid after you are admitted to a college.
Then it is completely irrelevant to your admission application.