After much juggling around, I finally found a tutorial which helped me the way through: Virtualizing a Linux System
Basically it's simple but needs a bit of work.
I needed to create images from all the existing partitions on my physical Ubuntu. I recreated a similar virtual machine and then I restored the boot loader. That's it.
AnotherHistorically, another method iswould be to use RemastersysRemastersys (much easier!) However, the project is now dead.