We don't have an ETA for this functionality yet, however, if you need to convert one shape into a somewhat different one you could try to:
Create your initial and final shapes in Sketch (or the tool of your choosing).
Copy the SVG codes (usually available in the right-click menu) for each one.
Paste them in a code editor, or even inside your design tool and copy the "Path D" values.
In Animator, place the playhead at the frame where you want your animation to start/end and paste the value from your previous step in the "Path D" property in the timeline.
Depending on how different the start and finish shapes are, the transition effect will of course vary. It might not suit all use-cases but maybe it's worth a try, since it's low-effort and easy to undo if you don't like the end result.
If your initial shape is very different from your final shape, you can try a slightly different approach:
Put the initial "Path D" value in the first frame and then update the “Path D” properties in later frames with “Path D” values from gradually different shapes created in Sketch until you reach your final shape.