There are many ways could you say this. Here are some suggestions: I was busy with my office work, and on top of that, I had an exam, too. I was busy with my office work, in addition, I had an exam (this is correct, but using "in addition" like this is something you'll rarely hear a native speaker actually say). I was busy with my office work, and in addition to that, I also had an exam. Last week, I was busy with parties and exams, and on top of all that, I also went on a field trip. Great question! :)

Both sentences do have the same meaning. I feel using "too" as opposed to "also" would sound more natural, however. Moving the "also" to after your verb (e.g "I was busy with my office work, [and] on top of that, I [also] had an exam) may also help. Hope this answer was helpful!