For example: she ahhed/ohhed/ummed/huhhed/hemmed. Or is this possible only with some interjections?
Some onomatopoeia are accepted as viable words. The few examples you give aren't among those commonly accepted. It would be better to put them in a quote, "She kept saying, "ahh, ohh," all the way down the street." Then again, when writing, you are quite free to use English in whatever way with which your editor will be comfortable.