As the title asks, is it possible to guess a privkey/generate an existing wallet's privkey?
That depends. If the private key that you are attempting to "guess" was generated from a non-random source (such as a passphrase or faulty random number generator) and you know something about how it was generated, then it may be possible to guess or generate the same key.
Blockchain.info briefly generated private keys using a non-random source due to a programming bug. An honest programmer has a computer running which constantly scans the network for such vulnerable private keys, and transferred the coins controlled by them to his own secure address. He returned them to blockchain.info with an explanation of the programming error it had made that left them vulnerable to theft.