Can an envelope feeder be added to the 9650?
No...not from from Okidata or any secondary supplier.
How does the quality compare on the Ricoh 420?
Ricoh has two 'speeds' - the slow speed (13 per minute) matches the Oki...but the Oki is around 50 per minute at full quality. The Ricoh can max 27 per minute at a lower dpi (600 x600, I believe). No feeder available either - just 50 or so at a time.
Do you know of any other options besides the Xante and PSI Envelope printers?
No. Both of these companies have their pluses and minuses. Xante has a larger presence in the market and can offer better service - PSI service is extremely poor. But the PSI has continuous feed. Cranking the Xante feeder up and down is a real productivity killer. With the PSI you can queue up a dozen jobs and never have to stop to fill up with stock.
Remember, both Xante and PSI lock you into their toner (you can purchase Oki branded drums, fusers, and belts, however)
BTW, the Xerox 7400 is a re-branded Oki 9650...also with proprietary consumables, both drums and toner. No feeder available.