Some pension plans allow employee contributions. These contributions impact the amount of the pension annual payment. These are pretax contributions, exempt from ordinary income but subject to FICA (just like 401k contributions). To represent these employee contributions, create a Special Expense and mark it as "excludable from AGI." This will treat it as pre-tax contribution (again, like a 401k contribution) without sidestepping the FICA taxation. The cash flow will be correct because the special expense sends the amount of the contribution out the door.
You should create a pension using the pension set up to account for the future receipt of annual pension payments.