There's a similar question about windows. This is the same, but for mac.

If I try to copy or move a folder to somewhere it already exists, it asks to replace it. That would result in deleting the target. Rather I want to merge.

There's already a aquataskforce request about this, and it's a discussion going for a lont time if it's even something that should exist on Mac, due to its whole philosophy. Discussions at apple are outdated and didn't help much as well.

As usual, there are professional solutions for doing this, such as Changes and Araxis. And there is the rsync or command line alternatives. But I want a free and simple solution, something like how it is done in Windows or Linux. I won't be doing it much anyway. By the way, PathFinder don't have such option as well and FolderMerge doesn't work on Snow Leopard as far as my 1 test went.