For OpenKM 5.0.x
Stop JBoss, open repository.xml file and find the "SearchIndex" section and add a new value in "textFilterClasses" and start JBoss again.
For OpenKM 5.1.x
Login in OpenKM as administrator and add the indexer in the "registered.text.extractors" property.
To implement a new text extractor refer to Jackrabbit documentation or take a look at OpenKM source code.