Hello Vinz9000,
The app is working only with PKCS12 file format.
The PKCS12 can combine the Private Key and Certificate Chain into single password protected, encrypted file.
There are free tools to do that, just search google for "how to combine certificate ca and private key into pkcs12".
Once the certificates and private key are combined into a PKCS12 you can go to
Settings -> Manage Accounts -> <Your Account> -> End-to-End Encryption -> Certificate Repository
Click on the + action button in the action bar and select the pkcs12 file in the file browser.
Thanks,
Anton.