I bought a pencil extender. It works great for graphite pencils, which are thinner. However, charcoal pencils are too thick for it. How can I use my charcoal pencils when they are too short to hold?
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 pencil extenders that allow up to 10 mm here.