If I import a private key into a bitcoin-qt wallet, is it possible that some day some transaction I send from the wallet will use its pair address (the bitcoin address created from this private key ) as a change address target in the outputs ?
As far as I know, Bitcoin Core (aka bitcoin-qt) always uses a brand new change address for every transaction, in order to improve privacy and make it harder to trace transactions. It never reuses an address that has been used before; I can't think of any good reason to do so.
With a little digging, one should be able to find the appropriate line in the source code; but I haven't done so.
On Bitcoin-QT can use whatever change address you want. Go to Settings->Options->Wallet and check "Enable Coin Control Features". Now when you go to send you can check "custom change address" and enter whatever you want, even if you don't have the privkeys.