Under Linux, you can use the isohybrid command to attempt to fix an ISO intended for a USB-stick, but that does not boot (because e.g. it expects a real DVD).

Warning and disclaimer: do make sure that you make a copy of your image file first and execute the isohybrid command on the copy.

The command to use is as follows:

isohybrid copy-of-your-image.iso

(Note: this requires syslinux to be installed.)