Apparently there's a way!
How to Convert a Bootable USB to an .iso File
Not exactly obvious (I'll give you that), but it does work!
Not only did I create that ISO file, but I also put it in my multibooting USB key, AND I just confirmed that it does boot the Acer Recovery tool too!
I have one Recovery USB drive with Windows 10 from Acer that I want to include along other USB drive sources into one using YUMI Multiboot.
I have already put 4 different utilities and Live OSs in that USB drive with no problem. But the source has to be an ISO file.
Thanks guys