Yes, you can still do that by using XTU. I am using a laptop which doesn't support undervolting in BIOS, but I have undervolted my CPU by ~125mV using throttlestop. You can use throttlestop instead of XTU as XTU is quite buggy. There are so many sites providing info on undervolting using throttlestop.
https://www.notebookcheck.net/How-to-Lower-Temperatures-Stop-Throttling-and-Increase-Battery-Life-The-ThrottleStop-Guide-2017.213140.0.html