Yes, but remember that .iso images are primarily for CD/DVDs, which use a different boot mechanism from regular disks.
If you have a "hybrid CD+USB" .iso image (like many Linux distributions provide), you can convert it without problems – just treat it as a raw dd image. For example:
qemu-img convert -f raw -O vpc Foo.iso Foo.vhd
VBoxManage convertfromraw Foo.iso Foo.vhd --format VHD --variant Fixed
But if you have an .iso that was not specifically prepared, and only made for CDs, then you will still be able to upload and attach it but you might won't be able to boot the VM from it.