You could set the cr to 0 and script the XP.
That's one approach I've used, when I've levelled up enemies to match the party level (since you can't alter their CR via script). In my case, I graphed the XPtable in Excel and fit a rough linear regression line to it, using the regression formula to approximate the XP reward. Although it'd be much easier to use a fixed XP amount by setting it as a local variable on the creature blueprint.
CR 0 also returns XP to certain player levels (1 to 8, I think). I wonder if you can set a negative CR on a creature blueprint? Or maybe make it greater than 40 so it won't be in xptable.2DA (assuming the game doesn't round things up or down to between 0 and 40).