I agree with sliedl. I have a customer who needed to create multiple A records and editing the zone files manually is the only way.
Remember to change the serial number, otherwise when you restart the DNS processes they will not recognise the new addition.
And, yes, the GUI is then off-limits for any future DNS changes.
Having said that, I have a vague recollection of being able to create multiple A records in the GUI. You just had to adust the case very slightly (server1, Server1, SErver1, etc...). The GUI would recognise the difference between the entries, but the underlying DNS server didn't care. I don't know if this is still the case now.