Assigning shape key to a curve To create a shape key of a curve we need to start with the curve's base geometry and then move the points in edit mode to have the new form, that's assigned to shape key.
Is there a way I can directly assign a curve to another curve as shape key? I can then draw the two curves (or maybe import them from an external file) freely and assign one as the shape key of the other.
Found the answer here:
https://blenderartists.org/t/blender-add-on-to-import-shape-keys-from-youtube/1124990
Here's the add-on that does this:
https://youtu.be/XMimQfQR_ss 
As per the suggestion in the comment, here's the brief description:
The add-on descried in the video above imports svg paths directly as shape keys. So if I have a rectangle and circle in the svg file and if I group them with rectangle at the top in the xml group, circle will be imported as shape key and rectangle as the target. Apparently, there are no limitations about the number of nodes, as texts can also be imported as a shape key. There can be many shape keys for one target.
Download link: https://github.com/Shriinivas/shapekeyimport