Agreed with Treemty.
Just take the + from both of the regular psp speakers and wire them to the + for the new center speaker, and take the - from both and wire that to the - on the new one.
It should power it no problem, but you're going to want an amp to make it louder.
What taking both of them does, is make the back speaker mono (as opposed to left or right only, it plays both) so that you've got your right and left (psp speakers) and then the big center one is mono, since it's neither left nor right.