It is now 2018, I've only just started with Proxmox. I have two Work In progress, sorry its not complete. Rough I know.
Method 1:
I'm going to use the Live CD method, but mount a VHD from virtualbox to the windows OS, it will look like a regular Harddrive. Then use free ImgBurn software to write the files and folders to an ISO. Hopefully that should be a sort of clone, I guess some raw data may get missed in the conversion.
Method 2:
I'm using VirtualBox 5.2 and migrating to proxmox. A graphical UI solution is :
1. Open 'virtual media manager' in VirtualBox.
2. Select the Harddisk of the VM you want to migrate vhd,vmdk,vdi ect.
Note: To test use a small file like < 8GB Raspberrypi x86.
3. Click Copy Icon or Right click, select copy . Copy Virtual Disk Image' pop up appears.
4. Select 'expert' mode, QCOW for the disk image type . Select where you want your copy /qcow file to go. click 'copy'.
5. Copy the qcow files for all your virtualbox VM disks to a USB drive big enough.
Note: is there a network way of doing this.
6. Attach the USB drive to the Proxmox physical baremetal server.
7. In proxmox create the VM , set the VM hardware to be similar to your Virtualbox Hardware.
proxmox equivalant drivers.
8. Figure out how to get the QCOW disk into Proxmox.