Hey Pierre,

On Fri, Mar 02 2018, Pierre Neidhardt wrote:
> An idea crossed my mind: from a foreign distribution with Guix
> installed, run
>
>      guix system init /mnt/etc/config.scm /mnt
>
> [...]
>
> Is there a simple way to do this without `herd'?

There is! I followed the vague instructions in this message to 
convert my debian installation to a GuixSD installation and it 
(broadly speaking) worked.

https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00139.html

The trick is not to "init", but just to "reconfigure" into GuixSD. 
It feels a bit hacky, but it works.

Carlo