Parametric Shapes are drawn when you use any of the Quick Mask Tools. To draw path-based shapes, you depress the ALT/Option key as you draw with one of the QMTs.
For specifics on your issue, you already have a path property. You will need to drawn an Ellipse with the appropriate QMT, with the ALT/Option key depressed. Voila! You now have two SHapes within the COntent prop.
Create a keyframe for the custom path and then move the Timeline Indicator to where the Ellipse should come into play. Copy the Path property of the Ellipse shape then select the custom path property and paste. You should now have two keyframes for the custom path with an ellipse path as the second keyframe. Switch off the visibility of the ellipse shape so that you have no redundancies.
It’s sort of a long way to get things done but it works and I don’t know of a faster way. FWIW, with masks on solids, one could use the Layer Panel to add (via pasting or drawing) additional mask paths to create shape morphs – off course the Mask Target menu had to be set appropriately. This procedures obviously saves a few steps but Shape Layers are Continuously Rasterized and that precludes opening them in a Layer Panel.
HTH
RoRK
Intensive mocha & AE Training in Singapore and Other Dangerous Locations
Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects CS6 ACE/ACI