Hi Kenji,
I'm not sure I really understand. Shape 01 is a comp, right? In this case your expression should look like this:
comp("MAIN").layer("Shape01").source.layer("target01").transform.position
And as you noticed, you can only access the position of the rectangle, not the positions of the corners... You'll have to calculate the corners positions from the position and the size of the rectangle.
If I misunderstood what you need, don't hesitate to send me a file, I'll take a lokk at it.
Cheers,
François