Hi there.
I think it’s not a good suggestion for all cases.
i.e.: chromeOS derived distro are .img, but inside there’s on a full GPT layout with several partitions for firmware, kernels, system data and the writable stateful area. That structure cannot be preserved inside an ISO without breaking boot compatibility across devices.
So conversion to .iso is impossible in these cases (I learned it in the hard way! 
).
rif.: Installing FydeOS on Your PC: A Step-by-Step Guide | FydeOS Help Center
<<Why not .iso?>> paragraph…
So, I’d convert a .ova to virtualbox, then try to import in qubes…
It’s really a lot of work for a simple test…
cheers!
M.