I agree with previouse poster, Josephine like a middle name could be an adorable way to get to Jo...
If you really want to use Jo, I can suggest you:
What about the name Jolene?
I was at lunch one day discussing names with a co-worker and a stranger came up and started giving input to me on names (maybe she is a fellow berry, who knows!). She told me a long story about her painful labor 40 years prior, fights with her husband over the name, a car accident, yada yada but it ended with her daughter having the name "Jo Clara" (Josephine Clara) and her granddaughter going by "Jo Lauren" (Josephine Lauren). Anyway, at the end of this long story, the point is this is one way to get to Jo that maybe is less "hillbilly."
My suggestion would be to go with Josephine, Johanna, or Jolene (as previously suggested) and use "Jo" as a nickname.
I like the Jo-Ellen suggestion. I also agree that Jo works best with short names that sound like/are nicknames to start with.
Or even a shortening like Roxanne suggested:
Josephine Beatrice - Joby/Jo-Bee
Josette Clarissa - Josie/Jo-C
Johanna Laurel - Joelle/Jo-Elle
I like the idea of Josephine/Joelle/Jolie as a middle name too eg. Primrose Josephine... Rosie-jo. I think it will give her more options as an adult to grow into. I really like suggestion of Ella Josephine Ellie-jo... Or how about
Lily- jo....Lilian Josephine
Ava-jo... Avalon Josephine
Milly-jo.... Camilla Josephine
Sunny-jo... Susannah Josephine (this one is my fave :)