importing users in multisite: should add existing users to the current blog

[pbiron]

When importing a WXR file in a multisite setup, if

1. an author in the WXR file exists in the multisite but not in the current blog; and
2. the "import author" route is taken (i.e., neither "create new user" nor "assign posts to an existing user" route is taken)

then a "Failed to create new user" error is reported.

Instead of that error, the existing multisite user should be added to the current blog.

I can create a patch that implements this if folks think it would be a good idea.

[mwillberg]

This is still an issue with 5.5.1 and quite annoyance when dealing with larger sites which do not have possibility to use wp-cli or other tools. The little I check the wordpress importer code it should be simple to add additional code to pull matching ID from the network when the cannot be created as it already exists.

If this has some potential issue with security, there could be an option "[X] Add existing users from the network and assign content to them"