Graphite greys are usually made from actual graphite, PBk10, so there's no direct substitute; you could try a dark mix of silver and black but it won't have quite the same effect.
Schmincke offer Warm Grey 1 and 2, made from different pigments, a number of which you won't have as single-pigment colours. Try a mix of Titanium White, a yellow earth (Yellow Ochre, Mars Yellow etc.), a little red earth (Venetian Red, Red Iron Oxide etc.) and a touch of black. If you have an orange you could try a light tint of that with a dot of French Ultramarine for a similar sort of colour.
Einion