Channel Mapping Options

I would love to be able to map a 2.0 stereo signal to a 2.1 system. This would feed both the left and right channel (combined) to the sub channel. This would solve a big problem for me. I would like to put a multichannel DAC in my system, but would lose the ability to have a subwoofer involved when stereo signal is being sent. I can solve it with Y-cables, but that erodes the signal. What do you think?