You need 502 plate appearances for a batting title. Nady only had 400 plate appearances in Pittsburgh, so he's not eligible.
It *is* possible to win a batting title in one league after being traded to another one, though. This happened with Willie McGee back in 1990, when he led the N.L. in batting average when he was traded. There, he *did* have enough plate appearances to qualify, so he won the title. The funny thing there was, he actually finished the season with a lower batting average overall (.324) than the second place finisher in the N.L. (Eddie Murray, with .330). McGee had hit .335 in the N.L., though, so he won their title.