Zhuyin IME and tone 0/5 syllables
Yes, you're right. The phenomenon of 豆腐 dòufu is the result of tone sandhi (连续变调 liánxù biàndiào). IME does not support tone sandhi, so you're unable to search for it as a neutral tone. The only accepted tone entry for 腐 is 3rd tone fǔ.