How to extend a circle during a trajectory ?

Hi everybody!

In order to make a waaaaahouuuu effect, I'd like to create this animation :

1- a circle is partly displayed on the screen

2- then, while starting zoomin, it moves in direction of the center of the screen

3- and then, when arrived at the center of the screen, it continues to grow until covering all the screen

In my old ages, I made this with dear Flash. It was called "shape interpolation" somethine like that... Any idea for making this in dear Storyline?

Lot of thanks for your help!