Here is a description of how to add sql functions by modifying the VoltDB source code: https://github.com/VoltDB/voltdb/wiki/Implementing-sql-functions
It is slightly out of date in its source code detail, but still gives a good description of the development process.
With possibly a little exploration in the product's git history to see how some of the modules have evolved, this should make it possible to add functions to the current version of the system.
Please let us know how it goes. We'd be happy to help you with additional pointers if you run into any problems. Also, if you are developing functions that may be of interest to others, we can discuss your signing up as an open source contributor and getting your code into an official product release.
Thanks for your interest in the product and we look forward to hearing from you again, soon.
Paul A Martel