Many thanks to both of you to have answered so quickly.
(Sorry I didn't give any news, I have been busy with exams for almost two month and I didn't even take the time to reply back, but your answers have been valuable to me.)
It seems you where right: when trying to build with "(pk ...)", I had the following output: ;;; (#f)
As suggested, I tried using the exact same procedure as in the shadow package, and the build was successful !
However, I could not really "test" the resulting build, as I only tried with perl and it took a really long time to cross-compile.
Do you want me to send a patch with the modifications ? I'm not really sure which branch it should go to.