open an old Knoppix.img with Knoppix 7

[user89]

I understand your "solution", a VM and so on, but that's not what I can use/need

I need to run Knoppix on very old machines that are not mine and that can't boot from USB key (old Bios)
so I have to use a DVD for Knoppix (except if sometimes they already have Debian on HDD for exemple, but I can not modify them) + a key for my data (and my /small/ softwares stored here are protected from any unwanted modification)
-- while til now, default Knoppix 7 scripts allow install of a crypted persistent container only on the boot device

the only "new" solution you give me is remastering, that I wanted to avoid if possible

[Werner P. Schulz]

.. and that can't boot from USB key (old Bios)

You can build a Knoppix Boot-CD: Summary
With it you can use a Knoppix flash disk installation as usual.
Perhaps Backup also may be useful

[Capricorny]

I understand your "solution", a VM and so on, but that's not what I can use/need

I need to run Knoppix on very old machines that are not mine and that can't boot from USB key (old Bios)
so I have to use a DVD for Knoppix (except if sometimes they already have Debian on HDD for exemple, but I can not modify them) + a key for my data (and my /small/ softwares stored here are protected from any unwanted modification)
-- while til now, default Knoppix 7 scripts allow install of a crypted persistent container only on the boot device

the only "new" solution you give me is remastering, that I wanted to avoid if possible

No, it does not seem to me that you understand this.
1. You don't have to use the same setup for transferring Knoppix images as for running later, as you seem to imply. We have not discussed the use afterwards at all, only procedures for conversion. You don't insist on doing the conversion on some very old machine if you don't have to???
2. You don't have to run Knoppix off CD/DVD, even it you have to use that for booting. So AFAIK, you can use a USB stick setup whether or not you can boot it. When you can't, you boot with the CD/DVD.
3. Remastering can be done VERY simply nowadays, so "avoid that if possible" is IMHO a very impractical approach if you want to use Knoppix with fairly extensive customizations in the form of added programs, as seems to be your case. The simplest for you is probably just to create overlays instead of doing a complete remastering. YMMV.
4. If you want to make extensive customizations, you simply can't rely fully on the default Knoppix install scripts - they are for the simple use cases. The "only on the boot device" conception which you easily get becomes very misleading - a basic functionality of Knoppix is the splitting of booting and running. For example, if you have a KNOPPIX directory somewhere on the HD and you boot from DVD/USB, Knoppix will use that image (the first it finds if several) unless you specify bootfrom= option on the command line.
5. The partition Knoppix is run from is mounted on /mnt-system, and on that partition, you can have as much persistent stuff as you like - it is always possible to automount it by simple modifications like I mentioned if system capacity permits. You can also have it on other partitions, but then there will be more coordination work. I have used several automounted knoppix-dataX.img files, put into the KNOPPIX directory for convenience - then they will be copied to new media by the Install Knoppix to flash script.
6. If you run the Install Knoppix to flash script, and let Knoppix create a new encrypted image for you on a USB stick, you have created the basis for what you need entirely with standard scripts. Using that for user data, and installing the additional .deb programs in noimage Knoppix instances, rolling them into overlays as needed, will give you a robust and handy setuo for your work - always using the stick, booting from CD/DVD when necessary. If you exceed that 4GB limit on the persistent image, my advice is that you either simply create more 4GB overlays as needed, or setup NTFS/extX partitions on the stick for larger files.

[user89]

I already told: the conversion is done (and yes I used an old PC to do that - Knoppix is for people who has less money too...)

[Capricorny]

I already told: the conversion is done (and yes I used an old PC to do that - Knoppix is for people who has less money too...)

Sure, Knoppix may be more relevant the less hardware resources you have got - but then to make the whole thing effective, it's important to organize the setup optimally. For example, I have noticed a huge performance boost using new, fast USB3 sticks instead of old, slower USB 2 on older USB2-equipped machines. Not to talk about the slowdown if Knoppix is not only booted, but run, from CD/DVD on such machines.