I'm trying to make an effect using the Blender Compositor, but I didn't know how to do it, and please I want the solution using the compositor not by adding more objects (Lens).
Here is an alternative that allows you more flexibility. You can use different textures for different effects. In this example I have only used the X axis for side to side distortion but you could just as easily make a spherical shape with the Y axis as well.
I have also shown with the Curves how to edit the amount of distortion.
The color ramp (which is inactive here) is another way to adjust the distortion effect.
I don't know your scene setup but mine is just plane with procedural stripes.
After render you just need to connect couple of sets - Lens Distortion + Scale nodes and adjust values as you like. X value will stretch pattern, Y is for straightening up after Lens Distortion.