The Parked Domains feature allows you to point a domain to your main site / public_html folder.
You can still use the redirect option under cPanel -> Parked domains and redirect a parked domain name to a sub-folder for your main domain. However, this will not keep the parked domain name in the URL. You will be redirected to a subfolder for the main domain.
In order to have a domain name pointed to a specific subfolder on your web hosting account, you should purchase an Addon Slot. You need an addon slot in order to have another web site associated with different domain name hosted under the same hosting account.