The user, which is being created while processing federated authentication request, is actually a virtual user. That virtual user is different then what Sitecore is having out of the box and what admin can create using User Manager.
Another point is, all existing Sitecore Membership users have their password associated with their account but in case of virtual user, there is no password as it is managed by external system and not by the Sitecore. Behavior of both type of user is different and that is why Sitecore has concept of Virtual user.
You can not map all those users with the federated users OOTB. If you really want to do that, you may need to customize the code by overriding ApplicationUserResolver class to consider your own logic to resolve external user.
However, it is not recommended to change this behavior.
If you want to map existing user just to retain their access rights configuration, i would suggest to go with OOTB way of virtual users creation in the different domain and mark those virtual users as persisted user by setting a value true in OWin configuration field called
<externalUserBuilder type="Sitecore.Owin.Authentication.Services.DefaultExternalUserBuilder, Sitecore.Owin.Authentication">
For more information about Virtual user creation, you can follow this link..