What add-ons are you wanting to migrate? Plug-ins should work, with the caveat that the Mac version is 64-bit only for CS6. So if you were running the 32-bit of CS5.5 with a 32-bit plug-in, then that won't migrate. Panel extensions might need a small modification. Scripts should work unless they specifically target CS5.5.
Are you having problems, or just trying to research before trying the beta build?
I am running the 64 bit version of CS 5.5 Extended on my Mac Pro Tower. My plug ins are Photomatix Pro and Viveza 2. I have downloaded the CS6 beta but have not started the trial yet, just trying to make sure that I don't screw anything up, so when in doubt, I always defer to the experts.
Thank you again.