1) If/After the POS will have been implemented, you will also be able to stake with LEOcoins that are in your back-office, as these are of course part of the blockchain. LEOcoin Foundation will announce updates regarding voting process (if any), date of conversion (if any) and availability of the updated LEOcoin Wallet.

2) Already currently LEOcoin wallets can stake (mine are!) and you will occasionally receive stake interest.

3) I have not heard yet if LEOcoins that are held on the LEOxChange will also return stake interest. I will try to get an update in this regard.