SSH and PuTTY keys are of different formats and will have to be converted to each other's format if you want to use the same key between the 2 programs. Follow these steps to convert SSH's key to PuTTY's.

1. Download PuTTY Key Generator (puttygen.exe) from the following URL.

   https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html

2. Launch the program and click on the Load button.
   
3. Set the Files of type to All Files (*.*) and select your SSH's key file.
   
4. Enter your key's passphrase if prompted.
   
5. A prompt will pop up once conversion is successful. Click on Save private key.
   
6. Choose the filename and folder to save the file to.