Yes, you can do it with bitcoin-qt, but not through the GUI.
Just create an unique label for the address you want to send from and run
bitcoind sendfrom <fromaccount> <tobitcoinaddress> <amount> [minconf=1] [comment] [comment-to]
That will not do what you think at all (accounts are *not* addresses).
The only way currently for the OP to do what they want would be to use raw tx's (and I wouldn't recommend that unless you really are very sure about what you are doing).
Oh, sorry about that, I was wrong.
I had to read the source to believe you, accounts should really be explained better, and sendfromaccount isn't a very good name then.
For the OP: Ok, so the only way to do it with bitcoin-qt is to do something like this:
Run listunspent, choose the outputs you want as inputs, do createrawtransaction, signrawtransaction, sendrawtransaction. Make sure you don't forget about fee or change.
There are scripts that can do this for you.
Or just export the private keys and use something like brainwallet.org if it's a one time thing you need to do.