Via spreadsheet and other services (Day One, Journey, Presently, Penzu, I Done This etc...)
1 up for Journey. But with that you would have to add some more things. For example more than 1 picture on an entry.
You can now import entries from Cactus.app. Are there other apps you would like to be able to import from? If so please not it in the comments -- thanks!
We completed the first step here. You can now import entries from an existing Reflection.app account. Which is handy if you want to migrate to another email or merge accounts.
Next step is to import from other journals or a CSV.