Back in the National League and reunited with general manager Jim Bowden and some former Cincinnati Reds teammates, newly acquired slugger Wily Mo Pena yesterday acted like someone who was just glad to be here. Which, in fact, he was. "I feel like I"m at home," Pena said after batting practice, during which he wowed onlookers, including his teammates, with some towering shots into RFK Stadium"s upper deck. "It"s a good team, it"s young, so from there we can take it." The 6-foot-3, 245-pound outfielder was having a tough season with Boston before Bowden, the former Reds GM who was trying to land Pena for years, pulled off a trade on Friday. In 156 at-bats with the Red Sox this season, Pena hit .218 with five home runs and 17 RBI. A career .256 hitter, Pena last year hit .301 with 11 homers and 42 RBI in 276 at-bats.