How Can I do that?
1) You will need to use the direct selection tool to cut the line from the original shape.
2) Use edit > paste in place (shift + command + v) to keep the line in it's original position.
3) Select the line and select effect > distort & transform > zig zag choose your options and select smooth from the points menu.
4) Use object > expand to convert the line to points and then use the pen tool to join your shape back together.
If you draw your shape with the Grid and Snap to Grid enabled, then you can proceed as follows.
In this example I assume you have already drawn the basic shape.
Select the Curvature Tool in the tool box, and add anchor points along the bottom line, then drag them into position. In the stroke dialog, set the corners to Round Join.