I have some altcoins in my Bittrex account. However, they are in my Bittrex wallet. I would like to create an offline wallet for Bittrex for safety. Is it possible? Can someone tell me the process?
No you cannot. Bittrex is an online exchange, it's not even designed for holding funds for the user. There is no way that you're going to be able to create an offline wallet, when you don't even have the ability to create and control your addresses.
If you need an offline/hardware wallet, either use trezor or ledger. They both have multi coin support.
However they do cost money and they don't ship to particular countries, so you have two more options. Either create paper wallets and store your private keys offline for every one of your coin holdings, or just download exodus which is trusted by many people to store multiple tens of thousands worth of cryptos.
Depends on your needs really, these are my suggestions. Hope they helped!