Neptune ships a powerful commandline application called remaster-kit which is able to remaster Debian or Ubuntu based distributions such as Neptune itself.
Remastering means rebuilding a system on the base of an already existing system.
There are dozens of possibilities to do this. The classical rebuild everything method extracts the complete live system from a distribution allows to do changes to it and repackages everything including the changes to a new ISO for CD/DVD or USB sticks.
Despite that remaster-kit also supports the Neptune specific live snapshots and merging changes from a persistency file to create a new ISO.
Remaster Kit is also able to automatically build ISO files.
If you want to know about remaster-kit take a look at the manpage provided with it. To start the manpage just enter man remaster-kit in a terminal.