The old way was to add some sort of behaviour to the unit which added experience.
Anyway incase you want the trigger method (added in a patch)...
const int c_unitPropXP = 24;
native void UnitSetPropertyInt (unit inUnit, int inProp, int inVal);
native void UnitSetPropertyFixed (unit inUnit, int inProp, fixed inVal);
native int UnitGetPropertyInt (unit inUnit, int inProp, bool inCurrent);
native fixed UnitGetPropertyFixed (unit inUnit, int inProp, bool inCurrent);
I think XP can be both fixed and int in value depending on how you set the veterancy behaviour.