install putty-tools
sudo apt-get install putty-tools
convert
puttygen id_dsa.ppk -O private-openssh -o id_dsa
links:
How to convert .ppk key to OpenSSH key under Linux
sudo apt-get install putty-tools
puttygen id_dsa.ppk -O private-openssh -o id_dsa
How to convert .ppk key to OpenSSH key under Linux