I have BitCoins in BlockChain - is it possible to transfer these into my CoinBase account?
You can transfer bitcoin from anywhere to anywhere! Simply log in to coinbase and click "Request" and then submit the blank form (this is one way to find your bitcoin address) and then send the coins there. Historically, coinbase has allowed selling coins as soon as they see them, not requiring confirms like most exchanges do.
One note about using web wallets or other types of deposit withdraw accounts is that sending coins between them is bad practice. If there is any chance your coins will be sent back by the receiving wallet, you should always use your own client, such as any you find on bitcoin.org.
The reason so is that exchanges and other web wallets will not necessarily send the coins which you had sent to your address in order to withdraw your funds. So if a coin will be sent back, it will show up on an address not registered to you and you will not be credited the coins back.