Ah, you've got me there Renato. This is a Gramps thread so I have to
defer to someone like Enno who knows more about what Gramps can and can't do.
In an ideal world, there would be event-types for such a registration, and a
workable way of adding custom event-types for the fake birth. This is all
something I'm working on but independently of Gramps.
In the Gramps/GEDCOM world the answer is easy. The registration is the
creation of a birth record, and if you store that right, the registration date
is the citation date.