First thing you’ll need to make sure that those VM images are using GPT and UEFI, then you’ll need to make sure that all the VirtIO drivers are integrated inside of them.
Once that’s all good, you’ll need a qcow2 version of that resulting image.
And you can feed that to LXD with lxc image import <tarball> <qcow2> --alias some-name
The tarball will likely just contain one file, metadata.yaml which is covered here Image handling - LXD documentation
In your case, you’d just need something like:
architecture: x86_64
creation_date: 1424284563
properties:
description: Windows XYZ
os: Windows
release: XYZ