This is a very risky proposition because of the risk that poloinex will recover your account back to you. I would think that it would not take much more than a signed message from a withdrawal and/or deposit address for you to have full control over the account.
How? In minutes? they will recovery in minutes? And why? I'm just want to reduce my losses..
How are we sure that you can profit out of it? You have a -6.5 BTC margin position and you want to borrow money to get out of the hole. Now what if you lose even more, just like the way you lost the first 6.5 BTC?
You can say it's because you have 2.5x leverage and with a 10 BTC loan you'll have 25 BTC that you can trade with in your margin account. Ok, but what if you lose 25 BTC before you make 6.5 BTC?
Then he'll run and you'll never see him again.
So I need about 10btc to leave the operations without loss.
You sound confident that you'll be able to make that money back. :-/