"HLOP" is a very strange way to abbreviate Opposing Force. It's more common to use "Op4" or "OF" :p
But back to the topic at hand, the base game does not provide any way to change monster relationship, but you can either use a mod/SDK that already provides this functionality as the base for your mod, or use a SDK with Op4 code such as
Op4 Updated SDK and either modify the Classify() method on the monsters or write in the functionality to override the Classify() value using a keyvalue.
The easiest would be using the
Featureful SDK as kimilil suggested, as it already comes with Op4 monsters and allows one to invert the monster entities' player relationship (i.e. from friendly to enemy and vice versa).