As the title says, I’m trying to find a way to match the edge of a SubD surcace (in white on the picture) to a circular curve (in black), or alternatively to a NURBS cylinder of the same dimentions, and I have not been able to find a way to do this operation.
Try the Align command and the To Curve option with the vertices along the open SubD edge selected. 8 to 10 points like you have should get very close to circular when aligned to the curve. If your ultimate goal is to join the SubD into a NURBS cylinder, do the above and then convert the SubD to a NURBS polysrf with the ToNURBS command. Leave a small gap an use BlendSrf after that to connect and join to the NURBS cylinder.
@qmalletdu The new TestSubDEdgeMatchCommand in Rhino 8 WIP should be able to help you match the boundary of the SubD to either the curve or to the NURBS cylinder you are describing. The test command supports position and tangency continuity. Check out this other post for more info TestSubDEdgeMatch: A new command to match SubDs to other Rhino objects .