Some cats can be more tolerant of others cats coming into the house than others. However the advice that's usually given is that it's best to avoid having more than one cat in one's home unless they're all from the same litter.
If Bella was hissing at other cats in her territory before, it's extremely unlikely that she'd welcome another cat now. Unlike dogs, cats aren't social animals and they don't welcome company - other than human company that is, of course!