While it's highly likely that Thirteen and the two other new characters will be dateable at some point in the future, nothing is yet certain. The three new characters are so new that they haven't even appeared in the main story yet, though there are new cards available where they appear in the associated Devilgrams.
To date their hasn't been any official word as to when or if the new characters will be added to the intimacy system. I do hope that it's sooner rather than later, though!