Technically, that would be a "Dive" not a "Charge". A dive doubles damage instead of adding +2 to your attack.
Dive doesn't require a feat to use - any flying creature can use it, and it can be combined with the Flyby Attack feat.
As for whether or not the Harpy is specifically capable of this action...
A Harpy has 80ft flight with average manuverability. She has to move at least 40 feet to stay in the air... no problem with this. The dive is 30 ft down to 5', so that's a 25' dive, during which she can move double her speed. She has no limitation on her dive angle, so it can be as steep as you like. After she dives, she has to move at 5' elevation for at least 5' before climbing again, at which point she has an upward angle of 60 degrees and half her speed. However, since she dove 25' at double speed, and has to move at least 40, she'll have more than enough movement left to get 30' back up in the air. The only question is really how much DISTANCE she is transversing.
In short, yes she can do this. Check out the flight manuverability table in the DMG. I took all the preceeding info off the SRD.