How to convert VMDK to VDI/VHD

You can use a two-step procedure then - first, use the clonemedium command to create a VDI image:

VBoxManage clonemedium disk aaaa.vmdk aaaa.vdi --format VDI 

(Have a look also at other options to clonemedium, like --variant. To read the help, just run VBoxManage | less or visit https://www.virtualbox.org/manual/ch08.html#vboxmanage-clonevdi).

Once you have the .vdi file, you can proceed with your modifications.

If you would like to convert only the virtual disk, not entire VM, you can also use qemu-img for Windows: it's free, portable and it runs under command prompt.

VirtualBox, VMware, Hyper-V disk image types are all supported.

For example, to convert VMDK to VHDX:

qemu-img.exe convert FileSource-Image.vmdk -O vhdx FileDestination-Image.vhdx

Extracted from: http://www.sysadmit.com/2016/08/vmware-convertir-vhdx-en-vmdk.html