No, they can't. Coinbase so far didn't credited BCH to their users. Coinbase announced that they will start to support Bitcoin Cash only in January. People who where holding bitcoins on Coinbase during fork, now have to wait until they will be able to claim their BCH.
Thats not whats being said
In the poloniex and coinbase announcements they both said they they will allow users to
withdraw their BCH but have not decided if they will open
tradingso basically when coinbase do give users access to their BCH they may just be to just withdraw to a wallet address and nothing else
this is only happening due to the risk of a class action lawsuit I dont think they have changed their minds they will still be against BCH