Dear @Guillaume_Laurent
your SubD has 12 Faces - and 2 starpoints, with 6 connected faces each.
the algorithm to convert ToNurbs has some limitations at starpoints.
you can not set the parameters for ToNurbs in Gh1 - but you can do it via scripting
see this topic:
in your case - because of the special, reduced typology a workaround is to subdivide the subd twice before converting toNurbs.
this has to be done with a script or with a plugin.
rhinocommon subdivide
hope this helps - kind regards - tom