IIRC Vendors that create xBase files honor the data structure for the DB only. Each vendor has their own index format. If you open the file with another vendors implementation the Data will be accessible however the index will not be (unless they support it).
Here are some of the index file format file extensions ndx, ntx, mdx, cdx and idx.

Jan Pazdziora has a index reading module on their web site at http://www.fi.muni.cz/~adelton/perl/#xbase However it does not appear to write index files, only read most formats.