I don’t own the 7900 but you can do it with an external crossover. You could pull the bars in the back that connect the preamp section to the amp and insert it there. Out of the preamp to the crossover then back into the amp input. From the crossover then out to the subs and from the home theater processor into the subs at the same time.
An active crossover like the JL audio CR1 will take both a 2ch preamp and home theater LFE at the same time. Something like a mini dsp might work too but I have not used one before.