Roll over of nifty futures

I have Rs120000 in my demat account and shorted nifty aug fut with margin of Rs10900 blocked. If on the day of expiry i want to buy back nifty aug fut and short nifty sep future on the same day. Will it be possible or i hav to add fund

You mean the margin blocked for August Futures is 109,000, right?

You can, as and when you square-off your position in August Futures, the margin blocked will be released. Given the sum if the margins you currently have plus those that will be released, you will have sufficient margins to take position in September Futures contract.