I'll assume you're talking about Conté pencils (Fusain Charcoal, that often come in sets with grey and white Conté).
Caran d'Ache has a pencil extender with a 9 mm diameter, which should be sufficient for those pencils, which are slightly smaller.
You can search for 'FixPencil 0012' or 'CARAN 12.009' (e.g. here).
And there are generic brand double-ended ...