Publication Manager has no extension possibilities.

More technical...

• The Search API, which offers a full-text-index over Solr, only allows to search for content and metadata of maps, topics and images. Publications are simply not indexed, hence cannot be searched and found.
• The Find API, which offers an exact database match over SQL, allows to find matches on metadata fields of maps, topics, images, templates and publications. It is this Search option which you see in the Web Client although technically driven by a Find operation.

On the backlog we have the feature to enable the Web Client's Search publication in Publication Manager (with its limitations of different search syntax as it is a Find and not a Search).
The feature to do full-text-index, so a Search, on publications is also on the backlog, but due to its price tag I don't see it making it to the top.

-Dave