In the Extrusion dialog window, there is a section for the degree of taper angle. If you don't have a particular second diameter in mind (or if you don't want to do trigonometry), you can add a taper angle to the extrusion of the first circle.Lastly, start the Loft command (Create -> Loft), and select the two circular profiles. Next, create an Offset Plane (Construct -> Offset Plane) off of the original sketch plane. Create the first sketch with the first diameter as you normally would. To ensure this, you can project (Sketch -> Project/Intersect -> Project) the first circle into the second sketch for reference. Take advantage of the Loft command The loft command is built exactly for this: creating a transitional shape. Then, create a sketch on this new construction plane, and sketch the second circle directly above the first one. The G0 (Connected), G1 (Tangent) and G2 (Curvature) options are available when the loft profile is the edge or face of a body (see: Loft Dialog Box). Beforehand, G0, G1 and G2 continuity options were available. Create the first sketch with the first diameter as you normally would. When recreating a 'Loft' feature between faces of two bodies in Fusion 360, it is only possible to select 'Connected' and 'Direction' as end conditions. Take advantage of the Loft command! The loft command is built exactly for this: creating a transitional shape.There are many ways to do this, but here are the easiest two:

Hollow loft fusion 360