You can add up to three 'email aliases' to your present account - these are additional addresses (not accounts) which deliver into the same inbox as the main account. (In fact it's a good idea to give out alias addresses, rather than the main address, because if they attract spam you can easily change them.) (New aliases can only be @icloud.com ones; @me.com addresses cannot now be created.)
You should be aware before you start that once you've created an alias you cannot turn that address into a full iCloud account, move it to another account, or reactivate it if you delete it.
More information on aliases here:
http://help.apple.com/icloud/#mm6b1a490a
Alternatively you can create a new Apple ID and use it to open a new iCloud account which you can sign into in System Preferences (or Settings)>Mail, Contacts and Calendars. This secondary account can sync data between its own online account and your Mac, though it cannot use PhotoStream. Data entered in one account won't appear in the other online, though both appear in alongside each other on your Macs or devices.