Michael Condrey has tweeted out confirming that Sledgehammer Games is working on adding a pause feature in Exo Zombies. Currently, there’s no way to pause the game at all, which makes it harder for solo players to reach higher rounds.
@MrRoflWaffles pause feature in progress. Complicated implementation needed for reasons I won't bore you with, but it's incoming.
— Michael Condrey (@MichaelCondrey) February 8, 2015
They haven’t provided an ETA other than “its incoming.” Stay tuned.