Pasenen, who can play as a centre back or as a full-back on either side of the pitch, has been bought in to give the injury-ravaged side more options.
Alan Curbishley was rumoured to be close to signing Pasanen for Charlton Athletic, and German side Werder Bremen were also in the hunt, but the out of favour defender has chosen Portsmouth instead.
Pasenen made his debut in the Finnish leagues aged only 15 with FC Kuusysi, and moved on to FC Lathi in 1998 where he was a fixture in the top flight club's defence. His success lead him to Amsterdam in 2000. Despite a very good start with the Dutch giants, his career at the Amsterdam Arena never recovered after his suffered a broken foot in 2001 and struggled to reclaim his place in the side.