Well it would be easier to just drop the old folder into the new one-- wait.
The new UTAU should have just combined with your old one if they were both installs.
Uh, anyway, if you have an aversion to combining the folders and you only had the ZIP version before, no, there is no way you can route it to pick up both voice folders. You can however manually load the old ones by going to Project Property> Voices > Other Voices> Bank Registry and then using the select button to load a new bank. Make sure to press Add.