You're fine. Dexron 3 and (most) power steering fluid are similar enough that a lot of cars actually call for ATF in the power steering (GM and Ford in particular).
Never use them interchangably on a Chrysler product though, their power steering systems use some unobtanium moon juice. Hondas are a bit picky as well, but can use Dexron short term.
If you're really paranoid, you can unhook one of the lines to the power steering cooler loop in front of the radiator and let it trickle out, then refill with Nissan power steering fluid (don't do this with the engine running, obviously).