Currently, you can add other managed accounts using whichever CommonName you like but they won't be assigned to any specific service or app pool (though you can do so manually afterwards).
BTW with regard to the services you mentioned... there is no good reason I'm aware of to use a different service account (than the general SP_Services) for things like UserCode/Sandbox, and the Timer Service must run as the Farm account. Be careful assigning new/arbitrary accounts to different services unless you know what you're doing :) There's a balance between the security isolation benefits of using multiple accounts vs. the additional load caused by multiple app pools consuming resources.
Brian
BTW with regard to the services you mentioned... there is no good reason I'm aware of to use a different service account (than the general SP_Services) for things like UserCode/Sandbox, and the Timer Service must run as the Farm account. Be careful assigning new/arbitrary accounts to different services unless you know what you're doing :) There's a balance between the security isolation benefits of using multiple accounts vs. the additional load caused by multiple app pools consuming resources.
Brian