Tony, Renato,
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.