Georges has written a post showing how to move a VirtualBox FreeBSD system to another, larger, VirtualBox drive:

“Let’s say that, like me, you once created a fixed-size virtual disk in VirtualBox, and installed FreeBSD on it. Now you’ve run out of disk space and you’d like to move your FreeBSD to a bigger-sized virtual disk. Here’s how I did it. This procedure was done with VirtualBox 2.0.{4, 6} on Windows XP.

First, with VirtualBox not running, make a backup copy of the whole .VirtualBox folder, just in case.

Start VirtualBox and select your FreeBSD virtual machine.

In Settings, Hard Disks: create a new Hard Disk, fixed-size. As the currently active HD is IDE Primary Master, the new HD will automatically be an IDE Primary Slave.When it’s done, click OK. The FreeBSD VM now has two hard disks. It’s like you’ve just added a new hard disk inside a real machine, a blank unformatted disk, which will be detected as a top-level IDE device (/dev/ad1) by the kernel.”

Go here for all the needed steps