1. The good thing about standards, is that there are so many of them

The vhdx virtual drive format as used in Microsoft Hyper-V is read-only when used with Virtualbox.

You can however convert the vhdx-file to something writable by Virtualbox.

Use Vboxmanage like this.

 c:\ > "C:\Program Files\Oracle\VirtualBox\VBoxManage" clonemedium disk <inputVHDX> <outputDisk> --format VDI

When you have your vdi-file, use it to create a new virtual machine in Virtualbox.

2. Sources

https://forums.virtualbox.org/viewtopic.php?f=9&t=83990

https://nerddrivel.com/2016/03/14/converting-virtual-machine-disk-formats/

Also see VDI to VHD: Convert Virtualbox vdi-images to Hyper-V vhd.