We can easily convert our ppk file to openSSH (for use in ubuntu and unix-like systems). For this conversion to complete you have to install something called puttygen(public-key generator for the PuTTY tools). This tool can interoperate with the private key formats used by some other SSH clients. Please read meaning of below options :
# sudo apt-get install putty-tools
#puttygen your.ppk -O private-openssh -o id_dsa
Specify the type of output you want puttygen to produce. Specify the type of output you want puttygen to produce. We chose private-openssh; Save an SSH-2 private key in OpenSSH’s format. This option is not permitted for SSH-1 keys.
Specify the file where puttygen should write its output. If this option is not specified, puttygen will assume you want to overwrite the original file if the input and output file types are the same (changing a com‐
ment or passphrase), and will assume you want to output to stdout if you are asking for a public key or fingerprint. Otherwise, the -o option is required.
Hope it helps someone.