Olivier Guillard wrote:

...

On a fresh installation that has not yet been configured: ...

Thanks howard, it helps.

For other readers I add this found in the slapadd doc :

...

LIMITATIONS Your slapd(8) should not be running when you do this to ensure consis‐ tency of the database.

Something I have to test :

What happens if you attempt to add an entry or change an attribute that already exists using "slapadd" (I suspect that the old entry is replaced).

No, that is not the meaning of "add".