What if we could add a new, unconnected person from the Attach Sources page? There could just be an alternate option below "Go To:" to create a new, unconnected person. It would save so many extra steps.
As a workaround, you can add the actually-unrelated person as a new child (or spouse) using Source Linker, and then edit the profile to delete the relationship. (It's what people do if they haven't found the "Add Unconnected Person" option at the bottom of the Recents menu. It's also the only way to add unconnected people on other genealogy sites, such as Ancestry.)