As per as Webcenter portal's certification matrix is concern, there is no direct way to integrate Apache Solr with Webcenter portal. It's better to use Oracle SES for your portal if you are intended to use webcenter's search taskflow's functionality.
However,you can integrate Apache Solr with webcenter portal as a J2ee application integration. You can very well create custom taskflow if you feel comfortable enough with Apache Solr. You need make call their client API and fetch the search results. But I would suggest do the data modeling and indexing very carefully to achieve relevant search results.
You can refer Drupal integration with Apache Solr in the following link..