can't add a new domain

Hi there, i want to add a new domain as i am changing from .org to .com and need them both to overlap for a while. zoho will only let me have one domain and says i have to upgrade to add another one. what is the deal with this, i already pay for 3 users and it is not cheap - i don't see what i have to upgrade to for a new domain or any other info other than a one line message. can you help? michelle