How can I import private keys to iSSH app in iPhone?
This is not the most user-friendly app. Besides importing the key, you should put MOSH support on for better connection on phone.

Step-by-step


Copy your key to Dropbox or any other service to copy the text to iPhone's default clipboard
General Settings > Configure SSH Keys > Import Key... > [Hold finger long, press Paste and insert password]
Add Configuration... > Use Key ON > Key > [Select your just imported key]