It depends. Are your speakers bi-wirable, i.e., do they have 4 binding post (one pair for tweeter, one pair for woofer)? If so, you may be able to bi-amp your speakers using the rear surround channel outputs from your receiver.
Some receivers (Panasonic, for instance) support "party mode" which sends the L&R front stereo signal to the rear surround outputs and it is designed to bi-amp a pair of bi-wirable speakers. I would also think that it would work if your receiver has a 5-channel stereo mode which sends the same signal to the L&R surround outputs that it sends the the L&R front outputs.
This will only work if your speakers are bi-wirable (have 4 binding post). Do not connect front and surround speaker wires to a single pair of speaker binding post. You could damage the receiver and/or the speakers.
Use caution, if you don't understand the bi-wire/bi-amp setup, don't attempt it.....