You can add new patterns via the pattern docker, or the pattern-quick-access menu in the toolbar.At the bottom of the docker, beneath the resource-filter input field, there are the Import resource and Delete resource buttons. Select the former to add png or JPG files to the pattern list.
Krita also allows you to make custom gradients and custom patterns. Since the location of my last explanation of custom gradients isn't obvious (it's in the layers management tutorial), I'm repeating it here:
Krita Patterns Download