how to create a 'keel' or a fin?

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?