ADD CPU to DOMAIN

Hello ALL,I need your clarifications.
We have M5000. working 1 domain with 2 CPUM and with total memory 32Gb . But 3rd CPUM is not using and staying ,waiting ))).
Because when that server came here, my friend was created domain only  with 2 CPU  by mistake.
Now how can I add that  3rd CPU without stopping the OS?

XSCF> showboards -a     
XSB  DID(LSB) Assignment  Pwr  Conn Conf Test    Fault   
---- -------- ----------- ---- ---- ---- ------- --------
00-0 00(00)   Assigned    y    y    y    Passed  Normal  
01-0 SP       Unavailable n    n    n    Unmount Normal  
XSCF>
XSCF>
XSCF> showdomainstatus -a
DID         Domain Status
00          Running
01          -
02          -
03          -

XSCF> showstatus