If your AIX server level is below 5.3 TL06, the easiest way is just to upgrade your current OS to TL 06 at minimum (take note it will depend of configurations for Power6 processors) then clone your server and install it on the new p6.
But if you want to avoid an outage on your account, you can do the next using a NIM server (this is not official procedure for IBM, so they do not support this):
Create your mksysb resource and do not create a spot from mksysb.
Create an lppsource and spot with minimum TL required (I used TL08).
Once you do nim_bosinst, choose the mksysb, and the created spot. It will send a warning message about spot is not at same level as mksysb, just ignore it.
Do all necessary to boot from NIM.
Once restoring the mksysb, there's some point where it is not able to create the bootlist because it detects the OS level is not supported on p6. So It will ask to continue and fix it later via SMS or fix it right now.
Choose to fix it right now (it will open a shell). You will notice oslevel is as the same as mksysb.
Create a NFS from NIM server or another server where you have the necessary TL and mount it on the p6.
Proceed to do an upgrade, change the bootlist, exit the shell. Server will boot with new TL over the p6.