Based on google search, most converter is limited with trial version or limited size that you can convert.
We come across a tool which is able to do the job for FREE. It's called :"WinCDEmu"
After install it, you just select a folder, right click and select "Build an ISO image"
Enter an iso name
Voila!. Job done...an iso has created.
Give it a try!
To know more, click here
To download, click here