position your mouse pointer on the outline near where you would like the new point to be and click+ drag the point into position, fine tune with the text boxes as you would any other control point. ctrl + click to remove points.
I'm not an expert - you might want to search K'tesh's posts - but I think this is probably impossible. To draw such a fin, though, you can attach them to a NARROWER component - for example a zero-length body tube - that sits just behind the tailcone, and attach the fins to that. Part of the fin will then be inside the tailcone, but this should be fine for Cp calculation. It could throw the weight/Cg calculation off, but you can adjust this by overriding the fin weight if necessary.
Yes, there is, but some trickery is required.
Observe below what I did to achieve the desired result with Nike-Hercules sustainer stage.
Add an 0.01 length 'fin support tube' of just the right diameter to level your fin with the cone.
Position your fins on the imaginary tube above
Adjust the fin shape to account for the sunken "fin tab" portion, so that that the are exposed to the airflow matches the actual shape of the fine.
In case of the FWD fin set, I had to "sink" the tail of the fin to match the height of the forward leading edge with the nose cone. Then I adjusted the height of the rear of the fin to match the desired shape.
In the case of the AFT fin set, I had to "sink" the forward leading edges, to match the height of the trailing edge with that of the tapered tail cone. Then I added 'fin tabs' to the front to raise the leading edge to match the height of the body tube. The rear fin height was raised by the length of the 'fin tab'.