What DAW are you using? You could always create an automation clip for the root and then create a seperate automation while bent to the note you want to bent to, copy the percentage of the pitch bend, and insert it in the original automation clip.
But as for specifically in Serum? I'm not sure what you mean. The pitch bend in the bottom right corner goes to exact semitones, or are you saying for a specific way to bend that to semitones linearly?