it seems those are raw img files. anyway there's not (yet?) a feature that "imports" raw images into or as VM disks
you could create a new VM (VMID say 100), with a new, small dummy, local or nfs storage RAW disk (say vm-101-disk-1.raw). then replace this file with the stacklet image file (untar/zip), but keeping the same name (vm-101-disk-1.raw).
start the VM to ensure your template is working, then you can live move disks elsewhere, and even change type to LVM or qcow2.
if you wish to use that VM as a template to build other VMs, convert it to template (right click on it in the left tree, iirc): once that is done, you can't use that directly anymore, but you can create other VMs, as full or linked clone...
Marco
