From an actual NACRA 18sq sailor:

Yeah, you can rig whatever headsail you want, but then you'll have to take knocks for it. "18 Square" stands for 18 square meters, which is the maximum legal sail area. The reason for the unirig has to do with maximum efficiency for limited sailarea.

When you install a jib, you'll lose pointing ability and won't have enough sail area to make it worth it for downwind. And you'll take a knock for it. If you just have to do something like that, at least install a chute which will help you downwind, and go away so you can point to weather.

But for DAMME sure if you have a stock NACRA Sq with the old-style "bendy" mast with loose diamond wires, DO NOT RIG HEADSAILS, and DO NOT TAKE ALONG CREW unless they're very light weight (kids, for example.)

The old-style (1980s, early 90s) NACRA masts will break, they are optimised for the boat and expected crew weight, and NOT designed AT ALL to carry headsails NOR extra crew.

All 18sq are NOT equal and you cannot generalise... keepup's is some sort of aftermarket or custom boat. newbie may have a NACRA Sq. I have a NACRA Sq with a newer 5.5u mast which has been lengthened. Husband has a Cat 1 Sq homebuilt with a carbon mast. Do you even really know what you have?

Squares are box-rule boats and usually are not alike so generalisations tend to be incorrect.