Is it possible to down-mix stereo outs to mono amp

I recently acquired an old Grundig Majestic console radio/turntable. It's an all tube mono amp with - as best I can tell so far - three speakers. With its FM section it's a surprisingly nice sounding amp, in a sort of lush tubey sort of way. It does have an unused input, but of course it's also mono. Is it possible to take the two channels out of, say, a DAC, and blend them down into a mono input? I'd love to be able to play streaming music or computer files through it.

Regards,