Like Sebi said, export, then import the private key.
I believe you know this post to withdraw the DAO tokens How do I convert my The DAO tokens into ethers using the withdrawal contract after the hard fork?
However, if you make any transfer it will be both an ETH and an ETC transfer because your transaction will find its way on the other fork and will be accepted as valid on both. Look up "replay attack".
So before you make any transfer, be sure to read about the splitter contract procedure. When splitting, you have to use the balance you had before you withdrew your DAO tokens (minus the gas of the withdrawal), in effect, your balance in ETC after the withdrawal, because that is the minimum balance agreed between the 2 forks.
Example with silly values:
Pre-fork Post-fork Post-withdrawal Post-splitter
(if 0.1 for gas) (if 0.1 for gas)
Addr1: 10 ETH + 1 DAO ETH-fork 10 ETH + 1 DAO 10.9 ETH 1 ETH
Addr2: 9.8 ETH
Addr1: ETC-fork 10 ETC + 1 DAO 9.9 ETC + 1 DAO 1 DAO
Addr3: 9.8 ETC
So after the split:
- the 1 ETH in your address1 is safe from replay because it has 0 in ETC fork
- the 9.8 ETH in your address2 is safe because it has 0 in ETC
- the 9.8 ETC in your address3 is safe because it has 0 in ETH