To simplify what is being said, yes this can be done. You would need to call customer service to have this completed.
A sub-account is like having two accounts in one. So if your main account has account #: 12345678-00001 then sub account 2 would have account #: 12345678-00002.
Each has its own billing and it's own plan so as was mentioned previously it will be more expensive as you will now be paying for a plan on each account (the main and the subaccount).