OMFG, seriously, this is like the third or fourth time somebody has come in here in the past couple months saying that they had some assignment that they had to use FreeNAS for yet the instructors don't know themselves how to use FreeNAS and they are telling them to do all these various things that are against FreeNAS 101, ZFS 101, and/or BSD 101. It's silly. You would think an Information Tehcnology professor would want to instill the values of "following best practices" in their students, not whatever the exact opposite of that would be called...
"Okay class your assignment today is to make a fileserver and do the exact opposite of everything you are supposed to do when making it."
Anyhow, you'll have to do some research to see what you need to do to get FreeNAS to recognize whatever virtual hard drive your VirtualBox is using, make sure the VirtualBox isn't pre-formatting the disks with some filesystem or another as well. You'll still need to have FreeNAS be able to see whatever hard drives you are using virtual or otherwise before you can use it with FreeNAS regardless of whether you are planning to put UFS or ZFS on it.
You may also want to try some of the older versions of FreeNAS as well, since the assignment in question I would be willing to bet was written back during like the FreeNAS 8 days, if not even during the 7 days (Which FYI is actually a completely different software that is now known as NAS4Free, perhaps the assignment is actually intended for what is now known as NAS4Free, but was written when NAS4Free was known as FreeNAS?)
There may be a few of the users around the forums though that have more experience with virtualization, and especially virtualization of FreeNAS that would be able to answer your questions better, I've never really played around with virtualization all that much so I'm not really qualified to be digging in much deeper than I already have here.
P.S. - Do me a favour and tell your professor that perhaps before assigning projects to students using certain operating systesm, mayhaps he should learn how to do the assignment himself/herself first. There's a thought. (You may want to wait until you get your final grade for the class before doing this though.)