how to create a 'keel' or a fin?

[Vishruth29587125tfx5]

I'm making a sort of shark/basic vertebrate model in 3D as a schematic.  Ive basically inflated a modified ellipse, to make a sort of 'apple mouse' shape, which works. I just want to add a snall dorsal fin/keel to the middle of the 3D model- sort of like adding a keel to a boat. Is there a way to 'pinch' the shape to do this, or should I make a fin shape and try to merge it to the midline? If so, how?

[Monika Gause]

You cannot merge these 3D shapes since they need to be created using different ways. You can only try and place it over the bode and hope it doesn't show too much. Illustrator is not a 3D application.

[Jlily98]

Can we see screenshots?