1. On the VirtualBox host, launch VirtualBox
2. Right click the VM to convert > Settings
3. Select Storage from the left navigation
4. Click the virtual hard disk and copy the Location value for the full path of the disk to the clipboard
5. Right click on the Start menu > Run > type cmd.exe > Press Enter
6. Enter the following commands in the command prompt

   # change directory to VirtualBox installation
   cd %programfiles%\Oracle\VirtualBox
   # convert the .vdi to raw .img
   VBoxManage clonehd --format RAW "pasted\full\path\to.vdi" "pasted\full\path\to.raw"

7. Once the .vdi to .raw conversion completes, open a web browser and navigate to the ProxMox web UI https://ProxMoxDNSorIP:8006/
8. Click the Create VM button at the top right
9. On the General tab, enter a VM Name and note the VM ID generated > click Next
10. On the OS tab select Do not use any media and set the Guest OS > click Next
11. On the System tab click Next
12. On the Hard Disk tab set the Disk size to 0.001 > click Next
13. On the CPU tab set the number of CPUs > click Next
14. On the Memory tab set the amount of memory to allocate in MiB > click Next
15. On the Network tab click Next
16. On the Confirm tab review the settings and click Finish
17. Select the newly created VM from the left navigation panel > Hardware
18. Click the Hard Disk to select it
19. Click the Detach button to detach the hard disk from the VM
20. Click the Unused disk
21. Click the Remove button to permanently delete it
22. Download WinSCP Download
23. Extract WinSCP and run the executable
24. Connect to the ProxMox IP server via WinSCP
25. Copy the VirtualBox created .raw file to a location on the ProxMox server that has enough free disk space, /root for example
26. Back in the browser, open the ProxMox host Shell
27. Run the following command to import the raw disk, modify the .raw file name and VM ID noted earlier

    # import the raw disk
    # usage:
    # qm importdisk <VM ID> /root/<source disk file>.raw <destination storage pool name>
    qm importdisk 100 vbox.raw HDD_500GB --format qcow2

28. Once the disk import completes, select the target VM from the left navigation menu > Hardware
29. Double click the Unused Disk > Click the Add button
30. Select Options from the left navigation sub-menu
31. Double click Boot Order
32. Check the Enabled box next to the hard disk
33. Drag the Hard disk up in the boot order as needed, typically below the CD-ROM device
34. Click OK
35. Click the Start button in the top right of the screen
36. Click the Console link to watch the boot process

Src:

https://i12bretro.github.io/tutorials/0295.html