You're in luck, because we have Arpasing! It's a recent method for English UTAU that does what you're asking for- easy conversion from English words to phonemes. For more information, like on how to use it, you can either go to UTAforum's resource section or check out the Arpasing website.
http://arpasing.neocities.org
You'll have to use arpasing voicebanks to use this method, which aren't very common yet.
A longer established english UTAU method is PaintedCZ's VCCV reclist, which has a series of video tutorials on YouTube here.
https://www.youtube.com/playlist?list=PLCplkjUb64gAUePNKBMRvv5sopzYd-77Z
There's plenty more voicebanks and support for this method, since it's been around for a while.
The fortunate thing is that you can convert VCCV voicebanks to Arpasing, which means that you don't even have to learn this arbitrary phonetic system. You can use the Arpasing Assistant plugin just as easily for turning words into phonemes. There's notes on how to do that on the Reclists page of the Arpasing website, and in the description of this demo.