You can add a sub but you'll be missing out on one of the main advantages, taking the lows out of the main speakers. That box mixer-amp doesn't have the needed routing options.
I can see why box mixers are attractive to musicians, but they're generally full of compromises like limited routing and poor channel eq. With a separate mixer and amp you'll have more flexibility for things like adding a sub, upgrading the mixer while keeping the same amp etc.