Hi Marco, so usually to make just one surface in a spot like that it's better to have initially made a double wide surface and then cut away chunks of it rather than trying to "tack on" a seamless piece like that.

It is possible to make a seamless surface in between 2 other surface edges using the Construct > Blend command, so it could be possible to use that in some way to build a smooth "tacked on" piece there.

One thing I'm wondering here is that maybe it would simplify things a bit here to focus on making half of it and then mirroring that and combining those pieces together possibly with blends to make them connect smoothly at the bottom.

- Michael