How to set password on already existing winrar archive and encrypt it also?
You can do it by using this method . Tools > Convert Archives > Compression > General > Set Password > then click ok and save . :)
This is not possible as the password is generally used as the hash/similar for the file, so, anything that changes it will require an intensive operation to change all files.
The fastest and easiest thing by far is to extract all files and create a new archive with the security set.
Not saying it isn't possible to do what you want, but I haven't seen the option to do it.