The best free ISO editor would probably be the giveaway version of UltraISO, which you can find here: http://www.raymond.cc/forum/freebies/12282-free-ultraiso-v9-full-license-key.html

If UltraISO's not enough to get the job done, there's always other free tools as well.

Totalmounter is a really good virtual drive app, with features never seen before: http://www.kernsafe.com/product/totalmounter.aspx

Imdisk is a pretty cool virtual drive program, which has one really cool feature which I've used before, remote mounting of image files and physical drives: www.ltr-data.se/opencode.html

You can then use the virtual drive as a source for UltraISO to create a new compilation.

There's also Imgburn, an excellent cd burning and creation suite: http://www.imgburn.com/

And last but not least, mkisofs, originally a unix-based iso filesystem creator: http://code.google.com/p/mkisofs-md5

Note: The link listed above will be the best one once it is updated to v3 of cdrtools.

Until then I prefer using this version of mkisofs: http://opensourcepack.blogspot.com/2011/11/cdrtools-mingw-binary.html

I really do wish there was a good opensource / freeware ISO editing app, but it doesn't look like there will be one anytime soon...