There are several web pages documenting how to add a zebra index to koha, e.g. http://wiki.koha-community.org/wiki/MRenvoize/zebra
Unfortunately, these changes must be made either to configuration files which are part of the Koha distribution (e.g. bib1.att, record.abs, ccl.properties and pqf.properties.) or to C4::Search.pm. As such, adding zebra indexes at a package site risks losing modifications upon upgrade.
I suggest that the files
As well as any files required for DOM indexing be placed in a configuration directory, e.g. /etc/koha/sites/* or $HOME/koha-dev/etc.
In order to be searchable, the index name must be added to @indexes in C4::Search.pm::getIndexes().
At the very least, there should be a way of adding extra indexes to the arrayref returned by getIndexes() without editing source code.
I've just opened a bug which is a partial duplicate of this one: Bug 12216
It only accounts for the zebra config files being over-ridden on a per instance basis, so I'll leave this bug open as a place holder for work on re-factoring Search.pm slightly to allow for customisation of the indexes array.