Sure, you can torn 25 fps into 50 fps, but if you expect to see a difference, you won't. When you step through the rendered footage frame by frame you'll notice that each frame is repeated. It's just the same as if you left it at 25.
Now, to get 50 different frames each second you can check out AE's Timewarp or you can use a plugin like Twixtor.
But if you simply put 25fps footage in a 50 fps comp, don't expect a miracle to happen.
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA