I just thought that if all storage can easily be "passed through" to a VM then it should in theory be very simple to boot the existing installation in a VM directly.

Regarding the extra storage: sharing disk space between proxmox and my current installation would imply that I have to pass-through "half of a drive" which I don't think works like that. Also, I'm using ZFS for my OS disk and I don't feel comformtable trying to figure out if I can easily resize those partitions without breaking anything ;-)