Yes, it is possible. But the system on virtualbox will have worst performance, and limited resources.
Not to mention that both operative systems will take resources at the same time
As i commented: Here is how to convert a partition to a virtual hard disk image.
Now, as you said, there is a separated /boot partition.
Can you reinstall GRUB? Relocating /boot to the new drive? If you can, then is the easiest solution. (I don't know how to do it, but is the easiest, because it only takes a couple commands, a google search may help you, as it may vary from distro to distro)
If you don't know, then you need to also make the old /boot avaliable.
If you also convert the old /boot into a virtual disk image, then you just need to set it in virtualbox settings.
Once ready, you need to boot the virtual machine (that you created) with a live CD.
Mount your "/" and go to the file "/etc/fstab" with your favourite editor.
You should know that the fstab files, says where are partitions mounted, so tell it to mount your boot partition on /boot.(You learn how to do it here, if you don't know what is your /boot disk, write "df" in the shell, and you should recognise it.)
Sorry if i didn't explained enough, but english is not my mother language. I tried the best i could.