Fellow spaniard here :). I think it's quite a different thing: a hikikomori is someone who suffers a depression, has coped with it by closeting him(her)self, and now doesn't know how to got out (nor does him/her want to, in many cases). Ni-nis are a very different phenomenon, a lot less homogeneous and, IMHO, caused mostly by lack of opportunities. The TV tried to present them as lazy rascals (I remember that infamous show from no less than a supposedly liberal channel like La Sexta...), but the reality is different.
There is, however, a risk that a Ni-ni may become a hikikomori, or something very similar.
Unlike Spain, there are opportunities, there's full employment for youth who want it in construction and other heavy work, but "Ni-Ni's" don't want to have anything to do with that.
There's a huge government "free money" project that's heavily criticized as a "Ni-Ni" enabler.
http://es.wikipedia.org/wiki/Ni-Ni
which according to Wikipedia is equivalent to NEET.
However, they're NOT shut-ins (many live in hovels, no place to shut in), so many become thieves and vandals.
Eighteen percent of youth in my country fall in this category (!!!!!)