I really doubt they'd give you a tour unless giving tours is something they do normally. Rules around visiting federal agencies, especially law enforcement agencies, are very strict after 9/11 and they aren't allowed to make exceptions to rules anymore.

You can call or email their public relations person, as Medie suggests, and see what options they have. I would not recommend just going there and showing up.

I have also had a lot of luck posting questions in the NaNo site's Reference section. You wouldn't believe how many people writing novels had past lives in really interesting jobs -- and I wouldn't be surprised if someone with familiarity with the FBI responded to your question.

Lorien