I have the same problem. I am trying to import data from Universum to Diarium. Universum also generates a zip file through its backup procedure, but the zip file is in a different format from the other apps that Diarium supports for "import from other app."
I read here in the forum that "Diaro allows importing from Universum, and Diarium allows importing from Diaro".
Personally, I don't like this solution because I have to transfer my data between different apps, and I'm not sure how they store it.