It was a feature in version 2 or 3, which I was using just for that for a while, but it was bugging out so had to switch to v4.
I miss it so much. My case has 9 fans and for each of them I have to manually tune a Custom curve each time. What I would like to do is set my curve once, save it as a preset with a custom name then apply it to other fans just by selecting it from a drop-down menu. Right now if I want to adjust my curves I have to do it manually over all 9 fans. It's beyond frustrating.
Hope you guys bring this feature back.