I think you mean topping up the pension for years you missed prior to joining?
I will check on the specific rules for the army pension. In most case in civvy-land, this is done through an AVC 'Additional Voluntary Contributions' or 'adding years'. However, the vast majority of forces staff never take out an AVC as the forces pensions are generous and topping up can lead to over funding - where your pension pot reaches it maximum contribution level and your additional money just sits there not working for you.
I'll get back to you on this. But let me know if I have misunderstood what you are trying to achieve.