VirtualBox - How to convert .img to usable VirtualBox format

VBoxManage convertfromraw --format VDI [filename].img [filename].vdi

Via.

VirtualBox - 64 bit option not visible in dorpdown

If you don't see "Linux(64 bit)" as an option in the dropdown, it means that virtualization is not enabled on the host.

Reboot the host, go in to BIOS and enable Virtualization. Exit the BIOS, making sure you save changes.