I've seen somewhere the solution for that, can't find it right now...
Try this, first you dump the private key and then you import it again:
To export a private key from your Satoshi bitcoin-qt client:
1. launch your bitcoin client as usual and wait for it to load the blockchain and start up
2. click on 'help' in the menu bar (top right)
3. click on 'debug window'
4. select the 'console' tab
5. type: walletpassphrase "your walletpassphrase here" 600
6. type: dumpprivkey [your Bitcoin address here]
7. this will return the private key, you can copy it now; ensure you clear your clipboard/history afterwards
8. type: walletlock
http://bitcoin.stackexchange.com/questions/4203/how-can-i-export-the-private-key-for-an-address-from-the-satoshi-clientImport key:
1. Run Bitcoin-QT
2. Select Help (to the right of Settings)
3. Select Debug Window
4. Select Console
5. If you encrypted your wallet, unlock it by entering walletpassphrase "YourLongPassphrase" 600 next to the > input box
6. Next to the > input box type importprivkey <bitcoinprivkey>
Example: importprivkey 5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS
http://bitcoin.stackexchange.com/questions/5941/how-do-i-import-a-private-key-into-bitcoin-qt