Hello Wakawakamush.
1-The back Image (and all 6 sides images) could be added using Upload sides Button:

2-The magicalvoxel .vox format needs to be tested. But I think it won't be possible because behind PixZels, the tech is not voxels. Voxels have a unique colour for each voxel, and PixZels uses 6 colours on each voxel (each side could have a different colour). That is done to create more precision when the user builds the models. (The models are nicer imo).
3- Obj with mtl it's a must, yes ... Many Thanks,
You can post your feedback here or send me mail (cartade1y0@gmail.com) I write down all the ideas.
Many Thanks for your feedback... That really help me a lot :)