You totally can deposit NMC and offload them for GHS or BTC. That's what I was doing with my NMC autopayouts when I was mining elsewhere with my hardware. And because people mining at cex are selling their mined NMC for BTC or GHS there's always a bit of it moving around in trades.

Bizarrely you can even deposit DVC and IXC even though you can't do anything with them in cex except hold them