Can I add a Buy Now to live auctions?

Yes, as long as reserve has not been met eg if you had a start price of $100 and your hidden reserve is $500 and bidding is at $400, then you can add a Buy Now as long as it's at least 1c above the reserve. In your items I'm selling page click on the Edit pricing button.

Then put in what you want then click the Update pricing button eg

If you are using the new site or a mobile app, use the Edit listing link and go through till you get the pricing page. Make changes and then double check all is correct before updating.