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