The new Photos.app does not come with Automator support (although there are third-party actions available). It does, however, have some basic AppleScript support, including support for importing a list of files.
However, I can't get this action to work correctly. Any attempts to use it, such as via the following simple script
tell application "Photos"
set theList to {"/Full/Path/To/Some/Photo.jpg"}
import theList
end tell
will cause Photos.app to report that "no metadata" could be retrieved from the file because it may not be a photo (however, the same file can be manually imported). Further investigation reveals that the automated approach is failing due to the lack of permission to read the file.
From the Console.app:
4/11/15 9:07:49.333 AM sandboxd[253]: ([3934]) Photos(3934) deny file-read-data /Full/Path/To/Some/Photo.jpg
(followed by a host of similar error messages). The third-party Automator actions I linked above have the same problem, for what it is worth.
How can I give the appropriate permission to Photos.app or the executing AppleScript/Automation workflow to perform this import? Note that the "permission" appears to be something other than (or in addition to) the usual file permissions, because setting the file and folder to "read/write from by everybody" (the nuclear option) does not change the behavior.