What age are you?
How much service do you already have?
What is your retirement age?
If you would have 44yrs service (or more) if you continued to work full-time, uninterrupted until retirement then jobsharing for 8yrs (assuming you'd be working half-time and a 1/80th accrual rate which is standard in the public service) would not seem to reduce your pension, assuming that yours is a final salary pension i.e. your 8 jobshare years would amount to 4yrs pensionable service and you'd end up with "full" pension on retirement.
If you would end up with less than 40yrs, you could consider notional service purchase (contact your HR dept) or AVC contributions. But don't rush into anything...lots of people are missing a couple of years at retirement and survive quite adequately; you'll probably need all your income while jobsharing so I wouldn't be rushing to plough it into your pension.