First, record an additional voicebank for your existing UTAUloid/UTAU library.
Second, oto it.
Third, add your suffix at each oto entry.
Forth, move your existing "core" voicebank into a new folder (but do not leave your UTAUloid/character/library folder, keep your character data there).
Fifth, place your new voicebank into your library folder.
If you add the suffix to a note, UTAU will render that note with the new voicebank. Also, there is a built-in prefix/suffix manager in UTAU which supports adding new suffixes. It can add your suffix at any selected note.
EDIT: Forgot to read the full post. There is a Questions and Tech Support subforum here.