I found a simpler way.

If you have a rigid physics object with a spring constraint behind the net then you get the net deforming while the rigid object absorbs the energy of the ball. Getting all the settings for physics and spring is tricky.

Here is an example you can look at. The rigid physics object is a dummy. Hit Cntrl-D to see it.

https://drive.google.com/open?id=0B3nsNVGroEz6UHJKZXRxb21GTHc