You can find examples on the forum of transitions made with plugins like curiloft.
Push/Pull Round to Square
Perhaps if you had an ellipse similar to the one in my effort, and loft a transition to a rectangle, you’d have a starting point for what you need.
The bit that I did there in the middle was just hand stitched with the line tool.
I’m sure there are others here more skilled in these “organic” shape than me.
Shep