Is there a way to convert a pbuilder base.tgz image to a .cow one?

I have this 862MB base.tgz image which is taking a considerable amount of time to decompress on every pbuilder invocation, I've read that there exist a cow alternative who keeps the files uncompressed so I'm willing to start using it, unfortunately I don't remember exactly the steps to recreate this image, so I'd prefer to convert it instead of starting from scratch. Is there any method to do so?