Well, I looked at the developers' scripts (all of them, except for those in the bonus missions, but there didn't seem to be anything like that there), and I didn't find such a function. I would recommend experimenting: come up with your own idea of what such a function would look like if it existed, based on what is known. By the way, regarding what is known: here are some functions I found that are not in the official manual (maybe they are in unofficial ones, I don't know):
SetUnitManaPoints(to whom, how much) {it seems that it was mentioned here}
setATB(to whom, position) {this is really interesting in my opinion; you can implement specializations for creatures using it instead of increasing stats, if you simply give a certain creature initiative in this way}
UnitCastGlobalSpell(to whom, which) {this has definitely been mentioned}
GetRagePoints(creatureName)
GetRageLevel(creatureName)
playAnimation(who, animation name, repetition)
Based on this, I can assume that the function you need, if it exists, looks like this: SetUnitAttack|SetUnitStat|SetAttack|SetStat
Possibly...