Actually, you have no need to replace default actors with these sprites to use them.
What you do is:
> Save them with a $ sign in front of their name.
> Import them into via the resource manager.
> You can now use your custom sprite and the original actors.
Saving it with a $ sign before the name lets RPG Maker know that your sprite is for only one character rather than a set.
Example below, copy below image into your project and you'll be able to see how it works.