I fail to see the point. If almost all your images are ISZ, then it's either of the following cases:
1) your "collection" of images is rather consistent - you don't create new images and you don't delete images;
or
2) you keep creating new images with UltraISO and maybe some of them deleting as well.
In the first scenario just convert all your ISZ to ISO (or whatever) and be done with it once and for all.
In the second scenario... well, you are already using UltraISO as your primary image creation software. Why not keep using it for burning as well. Why need to switch to ImgBurn, especially if you already paid money for UltraISO?
I don't trust burning support in Ultraiso. I had issues with verification (does not work at all in all versions). Also I don't want to convert ISO->ISZ because ISZ is smaller.
Also, guys, lets not talk about creation time of this thread, but about meritum - do devs plan to add ISZ support?