Whatever you do, don't sum the outputs of any electronic device through a "Y" adapter. Doing this can cause damage to the 950's output stage. It's OK to use a "Y" for inputs however.

I really don't know of any easy way to get a mono summed signal with the 950 alone. It is OK to sum the outputs through build-out resistors of around 4k7 ohms, but unless you can make such an adapter, you might have to search around, possibly at Radio Shack.