No — you can't link a demat account held with another broker to your Rupeezy trading account. Your Rupeezy trading account only works with a demat account opened as part of your Rupeezy account.
How do I move my holdings to Rupeezy instead?
Open a Rupeezy demat and trading account if you don't have one, then transfer your existing holdings from your other broker's demat account to your Rupeezy demat account via an off-market (DP-to-DP) transfer — your shares move, but the account itself stays with your original broker.