You should be able to add an "add-on domain". Look for it in Cpanel. It can get confusing in your server directory though since it won't be one file level below your main site.
I've started making my "main" domain a dummy name, a new domain name I don't plan to use, and then putting all of my sites as add-on domains.
Add-on domains are extremely common, so it isn't anything crazy.
I agree with Luke. Personally, we currently host 4 websites on one hosting plan with Bluehost.