> Is ist possible to convert a (round/interpolation) point on a curve to a edgepoint?

You mean for editing by Edit / Show pts?

There isn't currently a way to directly convert a point from a corner point to a smooth point, instead what you do is insert a new corner point with "Add pt" (either check the "Make corner point" box, or a shortcut is to hold down the ctrl key when you add the point), and then select and delete the old point.

- Michael