I've updated the code for openni-pcl and openni2 drivers to read from *.oni files. With the latest changes, you can set the file path in "device ID" box (under the Source/RGBD Camera tab in Preferences Dialog). The file must ends with ".oni". Also, if you recorded from OpenNI2, you should choose OpenNI2 driver to read the *.oni. For files recorded with OpenNI, you should use OpenNI. It seems that the oni files are not compatible between the OpenNI versions.
To convert *.oni files to *.db files, it can be done creating a data recorder in the GUI or using the console app: