you can use https://shapeshift.io/ (swap currencies)
As I see in OP it isn't about giving away anything. This is how the lending section here working. You deposit some alts (usually 120% of the loaned amount) at an escrow as collateral, you get your BTC and when you paid back your loan (+ interest) then you will get back your alts. In this case the lending site would act as lender and escrow in one. It will turn to an exchange only if you default on your loan .
To OP: I think this could work, but you would need quite strong security and just a set of well established altcoins as collateral to avoid issues like that what killed Cryptsy.