I would temporarily change the geometry of the highway so it does not cover the roundabout. Then create the roundabout from the central node.

After creating the roundabout, make sure all connections are OK, because there are missing a lot at this moment. Fortunately creating roundabouts adds connectivity as well in most cases.

After the roundabout is OK, change back the geometry of the highway.

If you encounter problems with creating the roundabout or connectivity, you might need to edit the levels of all roads that connect to the roundabout so they are al the same. Make sure the highway stays (at least) one level above the roundabout.