Boiling egg is a front-end of Egg (Tamago) V4.
You can convert roma-ji to kana without toggling input method.

Put the expression below into your ~/.emacs.

(autoload 'boiling-rK-trans "boiling-egg" "romaji-kanji conversion" t)
(autoload 'boiling-rhkR-trans "boiling-egg" "romaji-kana conversion" t)
(global-set-key "\C-o" 'boiling-rK-trans)
(global-set-key "\eo" 'boiling-rhkR-trans)

WWW: http://www.gcd.org/sengoku/boiling-egg/