Arguably the greatest frequent flyer offer ever came in 2002 when the KLM Flying Dutchman program offered not just to match elite status in another frequent flyer program, but also account balances (flights on KLM or partners were required for this). People earned millions of miles from a few flights.
Over the summer JetBlue matched Virgin America point balances with up to 75,000 bonus miles.
But I don’t recall an account balance promotion that was both unlimited (like KLM’s) and that required draining the points in your existing account.