You can -- but it's pretty challenging. You'll essentially be trying to keyframing Scale at the same time as you scale Cropping.
A better idea is to take the video you're going to use as a Picture-in-Picture and, in a separate project, use Scale to create your zoom. Then use Share/Personal Computer/AVI to output this scene.
When you then use that AVI as your PiP, it will appear that you're zooming into it without changing the size of the PiP!
I think that this would be easiest to create your PiP with Track Matte Keying, which will set it's "window" to a constant, and then you can Keyframe the Motion>Scale, as you need.
That's how I'd do it, rather than try to coordinate two Effects with Keyframing at the same rate and time.