If the mains aren't high passed, you could use the high level inputs on your subwoofer for its input signal. Given the proprietary subwoofer connection and spring-lever terminals for L+R hinting of low production costs, I'm guessing your mains probably aren't high passed.
Is this the back of your sub?
If so, just run a second set of speaker wires from your receiver's L and R channels into the spring terminals on the back of your sub, and there's a good chance it'll work. No, it shouldn't damage your amp. The speaker-level inputs on a powered subwoofer don't appreciably contribute to the load.