When I create a cv curve then pick cv -> curve edit > modifify > add point I can only add points at the end or the beginning of the curve....
I need something like "insert point" in Rhino: add a cv in a random position inside the curve
That inserts an EP and not CV. Essentially it adds a span.
One of the approaches could be to increase degree of a curve, rather than adding span & then rearrange CVs.
Insert tool adds both an EP and a CV. I use the Insert tool because you can see where the CV will be added and modify it's placement by dragging the insert handle along the curve. To see the resulting CV Layout before commiting, the CV's for the curve must be toggled ON. If you are concerned with the number of spans, then the rebuild option is definitely the way to go.