Yes, you can link your existing demat account with your ProStocks trading account. Also note that you can link multiple demat accounts with the trading account.
But we highly recommend you to open a demat account with us if you trade in Equity Delivery segment. This makes the transactions between the trading and demat account seamless and hassle free.
When linking to external demat account, when you sell a share through ProStocks, you have to manually transfer the shares to ProStocks.
Note that we offer Zero AMC Demat account to all our customers.