In the Warcraft III editor it was called "Acquisition Range". Apparently that got old, but I found the answer searching for that on this very forum.
Look up the weapon for the unit you wish to increase the "aggro" range of. Locate the "Stats - Minimum Scan Range". For a Marine's C-14 Gauss Rifle it's 5 by default (same as their attack range). Changing it to e.g. 15 will cause the Marines to spot each other earlier, walk 10 "range" to each other and then fire when they're in range. Suppose that's what you're looking for?