Disclaimer: Your old keys must be in DSA format. If they are not in this format, or you are not sure, generate a new pair of SSH keys.
In order to convert your ppk keys into OpenSSH DSA format (id_dsa and id_dsa.pub files) you will need PuTTYgen utility (PuTTY software). Open PuTTYgen, go to “File” menu and click “Load”, then open your ppk private key.
Open “Conversions” menu and select “Export OpenSSH key”. Save it as “id_dsa”. To export the public key, in the main window of PuTTYgen you can see a text box on the top which title is “Public key for pasting into OpenSSH authorized_keys file”. Select all the text in the box and copy it in the clipboard.
Create a text file with a notepad editor and paste the text into the file. Save the file and close notepad. Rename this file to “id_dsa.pub” and that is all.