One way to give an existing solid part a complex surface is to split the solid part using a face part as the splitting tool.

To do so you create your part slightly larger than you actually want it, but without the complex face. You can then modify the shape of the surface using the dynamic surface modification tools.

The edges of the face part must extend beyond all faces of the part to split.

You can either keep both split parts resulting from this operation or discard one of the parts as required.