This definitely sounds like a custom machining job. You'd have to have a sprocket made with a central hole large enough to fit around the hub shell, and either welded to the spoke flange or bolted to it with small bolts through the spoke holes. Another possibility is extending the existing range, by installing a double or triple crankset and front derailleur, with a chain tensioner to take up the slack. Alternatively you can fit two sprockets to the hub back-to-back, and shift between them using a rear derailleur.