Selling of altcoins

If an altcoin is added how can trading start?
How can altcoin holders sell in bisq? given one can’t import an existing wallet to bisq.

You use an external wallet. Create an account for the altcoin within bisq, enter your altcoin address… That is what bisq will show to the buyer to send funds to.

Yes, you would need to go to Accounts and create an altcoin account with the address from where you will be selling altcoins, I guess. Then you will need some bitcoins for a security deposit and then you can either create or take an offer where you will receive bitcoins by the end of the trade when you send your altcoins to the address that was provided to you during a trade.