When you load an ISO file, or extracted ISO folder, you will get the Create ISO option at the bottom of Apply settings.
If you started with an isolated WIM file, then you won't get that option (because the ISO files are missing).
In the instance you saved a modded boot.wim or install.wim, find an ISO & extract it, and replace the original WIM files under \sources directory with your updated files. If the extracted folder is Added to the Image page, you can select it and right-menu -> Create ISO.
Don't forget, the same disk drivers must be added to both WIM's.