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.