It's possible, but not very convenient.
Actually, if you search for it from bitcointalk, you will find some modified qt wallet can do this job for btc. However, pts-qt wallet doesn't have this function yet.
Currently, if you want to specify the sending address, the easiest way is to export the private key for that address, and import it into a brand new wallet(swapping wallet file or Virtual machine ). Then payments can only be sent from the balance of that particular address. But you'd better send all the amount in your address, otherwise, any remaining balance will be sent back to a new hidden address inside the wallet as "change"