Sorry, its been a while since I've looked at putty
. Is the .ppk just a public key file? If so, you should be able to (if you've copied the file to your home directory and that is your current directory):
cat filename.ppk >> .ssh/authorized_keys
Be sure you redirect with two
greater than signs (>>). If you use only one (>), it will wipe out anything already in authorized_keys.
If you want to delete or modifiy a line in authorized_keys, it is just an ASCII file so you can edit it with any text editor. Just be aware that the lines are quite long so a given line will probably wrap several times in your display.
Edit: When I speak of editing a line in authorized_keys, I am referring to the beginning of the line where you can add various options. Obviously you would not want to modify a key itself! See the AUTHORIZED_KEYS FILE FORMAT section of the sshd man page
for more info. If you don't need/want any of these options then you don't need to worry about editing a line.