There are multiple ways to do that in fcitx5-chinese-addons if you really want that.
I assume you're using pinyin in fcitx5-chinese-addons, otherwise, we don't support fcitx4 and other engine should go to support for the corresponding engine library.
Please explain why would you want that so we could figure an appropriate way for you.
Are you having issue about typing 成 as cheng? Does that happen to other character whose pinyin is cheng?
Are you only asking for this single phrase or all other similar phrase?