Web-E already gave a very good answer with the PowerISO tutorial.
As an alternative solution, I've found an interesting answer there:
It's for Windows, but as this solution is very easy, it probably worths creating a small Windows booting disk, or use the mini XP Windows included in Hiren's boot live CD. I don't know if it's feasible, it has to be tested to answer this question, but the software used here, imgburn is compatible XP.
Considering you didn't touch anything in your files or folders of your extracted ISO, as it's obvious you cannot create bootable ISO from any kind of files or folder (bootable files that will pass important info to the bootloader are needed).
In the example, they use ImgBurn (Windows) with "create image files from files or folders". Ignore the parts regarding Windows files (like ei.cfg) as we aren't trying to make a bootable Windows.
They turned a bunch of .dll files and other formats, to an ISO file, and checked “Make Image Bootable“ in the section called Advanced → Bootable Disc.
So, it's exactly what you want to do, turning a bunch of files and folders, with any formats, to an ISO, and giving the ISO the instrinsic "bootable" property.
Once again, it won't be bootable if the ISO from which you extracted the files wasn't already a bootable OS type.
There are other software able to do the same:
Winiso (their tuto is badly explained, but we see on the image that they used files and folders to construct their bootable ISO),
Alternatively, if everything fails, software like AnytoISO to create an ISO from any kind of files, and using another sofware to make the ISO bootable, it can be made from the command line, or from a software.
One way to check if your extracted folders are able to boot is to extract the bootable file from it. They use UltraISO for that here, “Bootable > Extract Boot File from CD/DVD”.
I didn't test, but I imagine that if there's not boot-file in your folders, it may return an error message.