My pension fund has about the same deal. The Illinois Municipal Retirement Fund (IMRF) allows an individual to put in extra money which IIRC, like the FCSRS, has the 10% limit, and IMRF also guarantees a 7% return, tax deferred. They keep your additional contributions separate from your normal contributions, then upon retirement you receive 2 separate pension payments per month.....the 1st being your normal pension, and the 2nd being from your additional contributions.
Unfortunately, I didn't find out about that deal until my last year of employment, while I was researching all of the details of the IMRF pension plan. At that point it was just a bit late to mess around with. I have, however, passed the info on to my former co-workers, and one
of them is now taking advantage of the 7% guarantee.....the others are investing most of their extra income in the adult beverage industry.