If bought with after-tax dollars, it's recovered the same way that other after-tax contributions to the pension cost are deducted. Pro-rated over the life of the pension, once you start collecting benefits. Usually there is some safe-harbor rule that allows this over all payments to age 80 or 85. Look up pension general rule.
If bought with pre-tax dollars -- for example, by transfer from a 401(k), 403(b) or IRA -- then it has already been deducted and you don't get to claim it again.
