In order to attach more speakers to your Expedition XP800 or Expedition XP1000 system, you would connect them to the Monitor Out section on your XP800 or XP1000 mixer.
This section is located towards the top right of the mixer in the Master column (Labelled as #9 in the XP 800 manual on Pg. 8 and labelled as #9 in the XP1000 manual on Pg. 9.)
These extra speakers must have their own external power source to work properly with your XP800 or XP1000 system.
Expedition XP800 and XP1000 Manuals: