Hello, and thank you for using the ADTRAN community support forums.
If you are asking about the ATLAS Redundtant AC Power Supply (PN: 1200220L1) which uses slots 7 & 8 in the chassis, then it can be installed with the chassis powered up. If you do this, I suggest inserting the module and tightening it down before connecting the power cord, but the main power supply does not have to be powered off.
If you are talking about a spare Main Power Supply (PN: 1200224L1) then the chassis can only have one main power supply installed at a time. There is no room for another.
Hope this helps,