How can I covert an ISO file to VDI and use it with Virtual box?

I have an ISO file that contains some backup files. I made it with this command.

• genisoimage -o backup.iso /var/backups

I want to convert this file to a .vdi file so that I can attach it to a Virtual Box VM. I did use VBoxManage convertfromraw --format to convert it to a VDI file, but when I attach it and power on the VM, it errors out saying "No bootable medium!". That makes sense because the VDI file only contains my backup files. How can I make that file bootable?

I was hoping that I could get an ISO installer of an Ubuntu VM and get the specific files that are used in the booting process, add them to my ISO file, and then covert it. But I don't know if this is possible or not. My final goal is to make a VDI file (form a custom ISO) that is bootable and can be attached to a VM.

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