Yes, however it will most likely take a very long time. I haven't actually tried the steps below, but they should work.
First, you need to make an image of your entire partition. You need to know the partition path of the Ubuntu partition, for example /dev/sda1 would be the first partition on the first attached hard drive.
sudo dd if=PART_PATH of=OUTPUT_PATH/ubuntu.bin
note that you should do the above command from a liveCD with the partition you are trying to copy unmounted -- i.e. you need to have another hard drive or something to copy all the data to.
You can shut the liveCD down and boot back into Ubuntu to perform the last step:
Convert the binary into a vbox drive:
VBoxManage convertdd PATH_TO_ubuntu.bin ubuntu.vdi --format VDI
you can then use the resulting ubuntu.vdi as a Virtualbox drive.
Just make sure you have plenty of time and hard drive space for this operation. A little tip: commandline tasks can be suspended by hitting Ctrl+Z. The operation will go to sleep, and you can use you computer again. When you are interested in resuming the process, type
fg in the terminal, and the operation will resume. Quite handy for long operations like this.