How to fill up a gap and create a smooth surface?

Hi All,

I am trying to build a shell for a part, and I started to do this by copying the surfaces from the original part and thicken them to get a solid. but when i try to add features, there is a small gap found between the thickened part and the new extruded feature. like attached picture. How can I fill it up and create a smooth outer surface?

Thanks for answering in advance.

Best Regards,

Kay