I have a .img file for Windows 10, and I need to turn it into something which I can use in VirtualBox, such as an ISO, or turning it into a bootable virtual disk for an ICT project.

Select a virtual machine by clicking its name in the VirtualBox window

Click the Machine menu at the top of the VirtualBox window, and click Settings

Click the Storage category in the Settings window

Right-click in the storage tree pane, and click Add Floppy Controller

Right-click the Floppy Controller device, and click Add Floppy Device

Click the Choose Disk button in the prompt window that appears

Navigate to the floppy disk image file (.IMG) on your computer and double-click it

If that doesn't work, try renaming the .IMG as .ISO and mount it.

If that too doesn't work, use VBoxManage's convertfromraw command as follows:

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

Mount the VDI as a hard disk.