perfmonk

Answering my own question since I found a solution.
You have to use the partition name for that command, not the LUKS mapping ...
Here is what I did :
(chroot) # cryptsetup luksAddKey swap /bt_keyfile.bin swap n'est pas un périphérique LUKS valide. (chroot) # cryptsetup luksAddKey /dev/mapper/swap /bt_keyfile.bin /dev/mapper/swap n'est pas un périphérique LUKS valide. (chroot) # cryptsetup luksAddKey /dev/sda3 /bt_keyfile.bin Entrez une phrase secrète existante : (chroot) # echo ### yes it worked! ####  
Sorry for the noise,
                      Bernard

Hey everyone,
I'm pleased to announce that Funtoo Linux and Sabayon (https://www.sabayon.org) are joining forces!
Now to explain what this means and how it came about.
Sabayon is one of the great Open Source projects and has an amazing reputation for delivering an easy-to-use Linux desktop that is built using Gentoo. For quite a while, Funtoo has been supporting the Sabayon effort because we believe in what they do.
Over the past several months, in chatting with the Sabayon team, it became really clear that we had a shared vision for creating a friendly and innovative community. We also were working on several next-generation technologies that were a good complement to one another. We decided it would be more powerful -- and fun -- to combine our efforts into one larger effort. So that is what we are doing.
What this means is that while each project will maintain its own personality, projects and management, we will be logically combining into a larger effort and there will be a tremendous amount of cross-project collaboration. This collaboration immediately multiplies our capabilities for delivering some next generation technologies that we have been working on and are excited to share with you.
The most immediate result of this collaboration is that moving forward, Sabayon OS efforts will be built on Funtoo. This, however, is just the first step. What I can say is that there will be much sharing of technology which will benefit both projects, and many new things.
For existing users of Sabayon and Funtoo, the communities you know and love will have new strength, support and ability to tackle a new phase of Linux Open Source innovation. I am really excited about this collaboration.
Welcome, Sabayon team!
Best,
Daniel Robbins